The edarly Myfords were distinguished by the centre height and centre distance.
the ML 1 was 3.125 centre height, and was 15" centres,, from memory (Dheck with Lathes UK )
The ML3 was 3.5" centre height, 15" centres, and the Ml4 was, like the ML2 a greater centre distance.
The thread for the chucl, originally was 7/8 BSW, (7/8 x 9 tpi ) but later was changed to 7/8 x 12 tpi.
Possibly, some of the very last were 1 1/8 x 12 tpi, but lacked the 1 1/4" register used on nthe ML7
A peculiarity is that there is no gear in the Saddle so that moves the opposite way to the handle rotation.
i.e..Clockwise rotation of the handle moves towards the Headstock
The gears ar 20 DP, 14.5 PA but are driven, (by driving collars on the spindle and Leadscrew ) or compounded by 3/32 pins, rather than the keys used on the ML7.
The bore and width of 7 Series gears is the same, so by drilling through an existing gear, it can be used as jig to drill a 7 Series gear for use on the earlier Mls.
But try not to drill right through, so that the opin is retained.
Threads will be 1/4 BSF , mostly with a few BSW included. The gib screws ar probably BA.
For full details, look at lathes UK pages on the Ml1, 2, 3 and 4.
It may be ols, but if in reasonable trim is quite useabl.
It bmight be nworth getting an extra 20T or two, so that a fine feed can be set up 20:60/20:65/20:60 on the Leadscrew. This should give a fine feed of abouyt 0.004" per rev. Yiou may need to fettle the spindle end of the banjo. to allow the stud to give good gear mesh.
The Tailstock barrel is prevented from rotating by a 1/4 BSF grubscrew with a key in the end that engages the keyway in the barrel. Once adjusted, it is secured by a locknut.
It might well be worth making an adaptor so that the later 1.125 / 1.25 register "standard" Myford fitting chucks can be fitted. I did this for somone who acquired a ML4, which we eventually got sorted and useable.
HTH
Howard