By 1944 the "Mk. 3" had made its appearance, its main identifying features being a completely new gearbox (with the gear-selector holes arranged in a distinctive V pattern across the front together with an unusual design of engagement lever) - and a pin release for the headstock cover instead of a large toggle lever.
Mechanically the headstock was almost unchanged, the spindle having the same bore and No. 3 Morse taper - but the previous safety feature of a separate cover over the large backgear bull wheel (under the main belt cover) had, unfortunately, disappeared. As an option, the spindle could now be ordered with a hardened nose - ideal if production work involving many changes of fitting was envisaged.
The shaft protruding from the changewheel cover was pushed in or pulled out to set the gearbox to generate fine feeds or threads.

A long-bed version of the 100 Series Mk. 3 clearly showing the gear-selector holes arranged in a V pattern across the front of the screwcutting gearbox.

On the 100 Series Mk. 3 the apron was still produced in two versions, 'Simplified' and 'Automatic'. Whilst the former retained the single star-shaped knob which was screwed in to engage power-cross feed - and used the leadscrew thread to provide a sliding feed along the bed - the automatic apron (shown above) was considerably modified with the 3-position selector lever replaced by a button. Pushing the button in selected power 'sliding', pulling it out power 'surfacing'; however, the drive was still through the same type of clutch as before - with no way of instantly disengaging the feed under power other than by twisting the screw-in, star-shaped clutch engagement handwheel and hoping that the drive stopped in time.
Very late examples of the Mk.3a have been found with the apron from the 200 Series lathe -identifiable by a large boss in the centre that carried the power feeds selector lever.

The "V-front" gearbox was entirely new, much more strongly built and used ball bearings throughout - however, it generated the same 48 pitches and feeds as before and used exactly the same (traditional Norton) method of selection. The first boxes had a bar topped with a black knob for the 3-speed top control - later versions used a flat, cast "thumb" lever.
A cross section through this box can be seen here.

The Mk. 3 tailstock was greatly improved (being the complete unit to be used on the new 200 model) with the main casting considerably strengthened, the barrel diameter increased from 1 to 13/16" and a proper (and rather large) "split-bar" barrel clamp fitted - no longer was it necessary to distort the tailstock casting around a slot in order lock the barrel in a set position - whilst the original quick-and-effective, toggle-type bed lock (which became a feature of later, larger Clausing lathes) was retained as was the No. 2 Morse taper centre.