I understand the performance was broadly similar. Changes reflected improvements ....simpler, cheaper to make, rationalisation etc. Basically if the lever bears on the camshaft lobe correctly it will work!
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Location: Stratford upon Avon

A catalogue states M used on small cars 32/33, Y with 2 1/4 diaphram on small cars 33 on to 1200cc. As previous the U and T used on some large engines, including trucks.

1 ½ to 2 ½ psi seems to be the normal operating pressure for most, with much larger pressure obtained from any pri lever. Different springs find their way into T types and affects the pressure (cars with crankcase vacuum had heavier springs). As doubtless aware the mounting spacing also determines pressure and ultimate delivery. Some cars have the wrong T lever with risk of slipping behind the cam.

Pressure mounted on car can be measured. For the adventurous, typically recommended 1.8 psi corresponds a column of petrol 5 ½ ft. engine cranked, not using any primer lever.

Location: Auckland, NZ