Manufactured by the Atlas Corporation in Kalamazoo, Michigan, for Sears, Roebuck & Co., the Craftsman 6" x 18" (3.5" centre height) backgeared, screwcutting miniature lathe was styled to closely resemble its larger brothers, the 10-inch Atlas and (almost) matching 12-inch Craftsman. Both the models sold with Atlas branding (known simply as the "Model 618" with 18-inch between centres and the "Model 612" with 12 inches) and the Craftsman version proved enormously popular, a situation helped by the usual efficient American mass-production methods and the availability of an extensive accessory range. Despite the model's popularity it was not, like the South Bend 9-inch, a lathe copied by other makers and only one example of a clone has been found, the Swedish Hogbo.
The numbering system used by Sears was unnecessarily complicated - indeed, it was not until around the mid 1940s that the system used in the catalogs began to partially match the actual model numbers. "Type designations" stamped into the identification tabs on these Craftsman-branded models have included: 101.07301, 101.20140, 101.2048, 101.2048F, 101.2120, 101.2140 and 101.M1518 (for a special mica undercut version for electric repair-shop use) and the very early (and rather different) Type 101.07300 and 101.0730, etc) that employed an integral countershaft unit. Should you have the chance to email the writer a photograph of the badge on your Craftsman lathe (of any age or type) he would be very grateful.
The 6-inch was first listed in a "Craftsman" version as the "101.07300", this appearing in the Sears, Roebuck catalog for the 1938 season dated 1st September, 1937. The Model Number was given as 99-PM-2045 and, astonishingly, versions of this were to remain in production (later in a Mk. 2 form) until the early 1970s.
Unfortunately no data about Serial Numbers exists - it seems not to have been recorded (or saved) by either Atlas or Sears. In addition, while its possible to obtain an approximate date from the numbers engraved on the headstock bearings of the larger lathes, these are not present (so far as is known) on any of those fitted to the 6-inch machines until around 1958. As the highest known Atlas Serial number is 027421, does one assume that figure represents the total for both Atlas and Craftsman branded lathes, or just Atlas? For a country with the population of the United Stated, 30,000 or so Atlas versions would seem to be a reasonable figure, so it's entirely possible that a similar number of Craftsman versions were also sold. 
Despite its modest price - at first just $42.50 without motor or changewheels - this was a very well specified machine with a proper tumble-reverse mechanism to the changewheel drive; a 1/2" diameter 16 t.p.i. Acme-form leadscrew (able to generate pitches from 8 to 96 t.p.i. with the optional-extra changewheel set); a dial-thread indicator; guards over the headstock belt and all gears; a 360° swivel top slide and 8 spindle speeds from 365 to a remarkably high 3225 r.p.m. Listed for only one year, the "101.07300", was fitted with a 3/8" bore headstock spindle with a 3/4" x 16 t.p.i. threaded nose, a No.1 Morse taper socket and a ring of 60 hole holes in the front pulley flange for dividing. With the spindle running in plain, off-the-shelf Oilite bearings it was obviously built down to a price and lacked a backgear assembly - and so was virtually impossible to use for screwcutting.
For the 1939 selling season the lathe became the 101.07301, the price increased and a number of improvements made including a 6.5 : 1 ratio backgear that improved the speed range to a remarkable 16 spanning a much more useful 54 to 3225 r.p.m. The spindle nose was increased in size to a much more sensible 1" x 8 t.p.i. (the 16 t.p.i. was far too fine for its application) the bore became  17/32" and the socket a No.2 Morse. With the spindle still running in plain bearings, the better spread of speeds was obtained by the use of a 2-step pulley on the motor that drove a 4-step headstock pulley - the eight speeds being (in theory) doubled by the use of backgear - though its engagement at higher revolutions to generate above 300 r.p.m. would have been unwise. (If the motor pulley is missing on your example - it's a pressure die-casting, relatively fragile and so easily damaged - the larger diameter was 33/16" and the smaller 111/16" - and intended to run a Z-section belt 10 mm wide at the top.) 
While the headstock front of both the two early types had a prominent frontal bulge, the same model badged by the makers under the Atlas brand was fitted with Timken taper rollers and had a much smoother, flatter front. Despite this move to "cheapen" the Craftsman version the same excellent speed range of was claimed for it as the "genuine article".
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Craftsman 6-inch lathe in its original, 1937 form that was to continue in production (virtually unchanged) until the early 1970s

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Although most small contemporary lathes in both America and Europe were sold either without a countershaft, or required the owner to purchase a separate bench or wall-mounted unit at considerable extra cost, on the first Atlas 6-inch this vital assembly was built in. Pivoting from two lugs on the back of the lathe bed, the countershaft had its belt tension provided by a long rod screwed into the casting that passed forwards through a hole cut in the face of the headstock. The system (crude, but effective), was arranged to work in exactly the same manner as that employed on some versions of the Craftsman 12-inch lathe where two rings were fastened to the tensioning rod with the outside face of one, and the gap between the two, forming location points that allowed the bar to be pushed back and pressed down into the hole to tension the belt - then lifted and pulled forwards to release it. To give some degree of adjustment to the final tension setting, the other end of the bar, where it was threaded into the body of the countershaft, could be screwed in and out. Only early versions of the 6-inch were equipped with this system - and are now rare; should any reader encounter this, or other novel versions of the lathe, the writer would be pleased to hear from you.
By 1939/40 the countershaft for the standard lathe had become a separate unit that bolted to the bench behind the headstock with a neatly-designed swing head to adjust the belt tension. A stand was offered in the form of cast-iron legs joined by a substantial timber top with sufficient room to mount the proper maker's countershaft and motor assembly.
Although the Atlas-specification roller-bearing headstock was a much more expensive proposition than the plain-bearing Craftsman version, during the 1940s and 1950s some Craftsmen 6-inch lathes were sold with what amounted to an Atlas headstock - a result, perhaps, of shortages of plain-bearing units or as an experiment in marketing - the clue being that the Atlas and Craftsman casting were slightly different. One can imagine the pleasant surprise of earlier Craftsman customers as they unpacked their machines to discover a "roller-bearing Atlas" for the price of a "plain-bearing Craftsman". However, by 1959, the Craftsman's catalog listed the 6-inch as being fitted with Timken roller bearings as standard - though it seems they had been fitted from as early as 1957. It appears certain that if a lathe is carrying the identification tag 101.214**  it will have a roller-bearing headstock. The bearings will be Timken - both outer races marked 07196 and the larger inner 07100 and the smaller 07079.
Many of the lathe's castings also carried standard Atlas "M6" numbers, including the designation M6-2 cast into the headstock of those models fitted with roller bearings. Other parts were marked: countershaft assembly M6-20B; bed L9-1; tailstock M6-5; cast cover at the rear of the cross-slide M6-37; headstock belt cover M6-22; changewheel cover hinge M6-85 and changewheel cover M6-28.
Although the two machines are very different, confusion has arisen in the past between the Atlas-built 6" Craftsman with another lathe sold be Sears, the "Craftsman 80" made by AA (the "Double A" Company). The AA was a very inexpensive, almost inadequate machine and cost, in the mid 1950s, as little as $48 compared to the $160 of the very much better Atlas/Craftsman.
Introduced in the early 1970s with a distinctive "square" styling (1972 seems confirmed as the year), the Mk 2 version of the Craftsman 6-inch was identical to its Atlas brother and dispensed with the speed-reducing countershaft - the 8-speed drive being taken directly from the motor by a tensioned V-belt to a multi-step pulley mounted outboard of the left-hand end of the headstock spindle; a picture of the Atlas version can be seen here..

Craftsman 6-inch 101.07300 without a backgear assembly and with belt tension provided by a long rod screwed into the countershaft casting and clipped into a hole cut in the front face of the headstock.

Integral countershaft and motor mount of the early (1937/38) Model 101.07300 Craftsman 6-inch lathe. The long belt-tensioning rod can be seen emerging from the back of the headstock casting