American Long Taper with Drive Key and Draw-nut Type L
Widely used on industrial-class lathes from the late 1930s until the 1960s.
The spindle nose is provided with a long but steep taper for centring and locating fittings, a key to provide positive location and a flanged nut for retention.
CLICK HERE FOR A PDF of Spindle Nose Fittings

American Standard Type A-1, A-2, B-1, B-2 etc. Short taper with bolt or stud fixing.
Commonly used on turret lathes, single-spindle automatics and larger industrial-class  ("Engine") lathes.
Type A-1: tapped holes on outer-bolt circle Co and inner-bolt circle C1.
Type A-2: tapped holes on the outer-bolt circle Co but no holes on inner-bolt circle C1
Type B-1: drilled holes on outer-bolt circle Co and tapped holes on inner-bolt circle C1
Type B-2: drilled holes on outer-bolt circle Co but no holes on inner-bolt circle C1

American Short Taper with Cam-lock Fitting Type D1
A replacement for the original American long-nose taper and widely used on many different sizes of lathe.
The different sizes are referred to as D1-3", D1-4", D1-5", etc. although some publications and parts lists have the figures are reversed as in 3"-D1 or simply list them, as in the cart below, as 3" (3-inch), 4" (4-inch) etc.
The spindle nose is provided with a short tapered flange of the same dimensions as the American A-1 Type but has in its face either 3 or 6 holes (for light and heavy-duty use respectively) into which the mounting studs on the back of a chuck or backplate can fit. Each stud is cut away in a semi-circle half-way along its length and an eccentric cross pin, turned by a standard chuck key, causes these to be drawn inwards until the pin goes over-centre and locks. The pins, being screwed into the back of the chuck, can be turned until the locking action is positive.

American Short Taper with Cam-lock Fitting Type D1