Senior M1 and major Millers invariably contain a "lathe-like backgear" assembly inside the main column. This useful mechanism enables them to be used to the limits of their strength and capacity in the horizontal mode - yet allows a usefully high range of speeds to be retained for the (simple type) of vertical head driven directly from the spindle nose.
A spring-retained pin in the face of the large bull wheel is drawn out to decouple the gear and pulley - and the lower gear slid along the shaft to engage it.

Power feed to the table is taken from a pulley mounted on the end of the main spindle. A Y-shaped arm functions both to tension the belt and keep the drive behind the main column where it can be safely contained inside the large, hinged rear cover.

The lower pulley in the illustration receives power from the column-head mounted motor. It is mounted eccentrically and can (by pulling up the knurled knob) be rotating so that the belt tension is released.

Worm box in the engaged position - the cover illustrated below is need to restrict its movement.

A bronze wheel on the table feed screw engages with a steel worm - contained within a housing which pivots on a pin in line with the universal joint.
The inner of the two arms on the worm box is retained by a spring-loaded pin which enables the gear to be engaged and disengaged from the wheel. The mechanism is prevented from dropping into the position illustrated by a cast aluminium cover - see below. The "exposed" gearing was used on early versions of the M1, later models used the arrangement on the Major where the worm-and-wheel gearing was provided with an oil bath.