Lathes UK website is a mine of information.
Briefly;
ML1 and 2
3.125" Centre height. ML1 is 15" Centres, ML2 is 24" centres.
ML3 and 4 3.5" centre height, ML3 is 15" centres, ML4 is 24" centres.
Not all have tumbler reverse (this was an optional extra)
Mandrel thread was originally 7/8 x 9 tpi (Standard 7/8 BSW), this changed to 7/8 x 12 tpi, and the very last ones , I believe, had 1.25 x 12 tpi, but lacked the 1.250 register which became the standard when the ML7 was introduced in 1947.
Secondhand machines may not adhere rigidly to spec, possibly having been modified since being built. So you could have a bed with an early serial number, but a much later, or even different, Mandrel fitted.
Ex factory, Mandrel and Tailstock were 1 MT.It is possible that some owners may have modified to 2 MT
Threads will be BSW and BSF. Gib strips may be BA.
The Leadscrew should be 8 tpi, but Cross and Top Slde leadscrew were 12 tpi, the dial having 80 divisions, so that each increment is slightly more than a thou.
Changewheels are 20 DP, 14.5 degrees pressure angle, but are driven / compounded by 3/32" pins in holes drilled partway through . Gears from the later 7 Series will fit, but will need the 3.32 hole to be drilled. (this will probably mean drilling through one of the existing changewheels to act as a drill jig. If you do this ensure that the hole is well away from the keyway.
The Tailstock cross travel adjustment is not of the best design, but was improved. It is "liveable" with care.
HTH
Howard