According to AC data the 1932 Austin 7 used 856728 (M)and it's replacement exchange unit was 7950221 (U) in turn this was replaced by exchange 7950221@ (U with a priming lever).
From 1932-july 1934 the pump was 856841 (M ) and its exchange unit was 7950221@ (type U)
From July 1934 The to 1939 the original pump was 856964 (T) again it's replacement exchange was 7950221@ (U)
In summary 7950221@ replaces 856728,856841,856964 and 7950221
As an aside & to make life more interesting(!) the early Austin 10 (1932 to engine 7470) also used the seven's 856728 (M) and IT'S replacement exchange unit was 856965@ described as type T & U. This replacement exchange unit fitted virtually every Austin engine (EXCEPT the Seven) from 1933-1939, and even on to the 8 & 10 of 1945-47. Including all the trucks & coaches from 1938-1948 - although the original AC fuel pumps fitted had a myriad of numbers.
This does not mean that because the 1932 seven shared a pump with the early ten and that pump could trace its direct descendants down to almost every Austin engine fuel pump until 1948 that this super common pump is interchangeable! There may be variations on inlet & outlet positions, size & valve types although the diagonal mounting stud hole orientation should be the same. Some pumps will be considerably larger, and may not fit the space on the seven engine....which is why AC specify different pumps for the seven to "the rest".....
Note that the T pumps had a bell shaped top cover (& separate valves & springs)) the U has dome top & I think integral valves & springs. I don't know if the rocker arms are all the same for all the Austin engines...
I hope this is clear!
Note where I use the @ symbol AC use an lower case 0 with a diagonal through it; I cannot do this on my keyboard. All this data taken from the 1962 AC Factory Exchange Services 16 page booklet. This also said that a priming lever could be fitted to any Factory exchange unit not so equipped for an extra 7s6d List!
E & OE Excepted!!!
David, Thanks for the comprehensive reply. What I have been trying to find out is there any difference in the performance, pressure/flow rate of these pumps. Provided they have the correct lever fitted they all seem to work OK. So why the changes?
I had noticed that early Type 65s(Nippy) seemed to be fitted with the M type pump, but this could have been changed in the last 80 years!
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!
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.