If I can address various points from various posts:
The parts Bazyle linked to under 'this is not what it says' are not Myford. The seller tells me that he assumed them to be Myford because they were in a box with an ML1 bed.
The '80 thou per turn' dial was an approximation by Myford to the 83.3 thou actually given by the 12 tpi thread on ML1234. Early lathes didn't have a dial at all. There was a seller on ebay making resettable dials with 83 divisions, but I haven't seen them advertised lately.
From my researches(and I am perfectly willing to be challenged) I believe that the ML3 was the 'Standard' version of the 'Superior' ML4 with the same centre height, bed length, mandrel threads and other major dimensions. The only differences I can find are that until 1938 the ML3 headstock was cast in rather than bolted on, and that it used plain bearings in the cast iron head rather than the ML4's bronze bearings. Some people favour this arrangement as it lasts longer. The ML3 was discontinued in 1941.
Accordingly I believe that any ML4 part should fit the OP's ML3. The topslide thread, handle and bracket shown in the link are attractive because they are late ones with the cast bracket bearing an engraved line to aid reading the dial. Earlier ones had a flat plate with no marking at all.
The ML1 and ML2 were smaller versions of the ML3 and ML4, and I believe that their history ran in parallel. They had a 3 1/8" centre height (compared with 3 1/2" ), measured 15" between centres (compared with 24" ) and had 3 1/2" cross slide travel (compared with 4 1/2" ). All early ML1234 lathes had a choice of a 7/8" x 9 tpi or 7/8" x 12 tpi Whitworth mandrel thread. The later ML4 (and I presume ML2) had a 1 1/8" x 12 tpi thread identical to the ML7 but the register was 1 1/8" diameter, not the 1 1/4" of the ML7. This means that ML7 fitments will thread onto the ML4 nose, but do not engage with the register. It should be possible to secure a collar to the existing register and turn it to the ML7 dimensions, making it truly interchangeable. The later ML4 was available with tumbler reverse, though I don't know when it was introduced or whether it was available on the other models.
I hope this clarifies some of the puzzles.
George
Edited By JasonB on 04/09/2016 13:43:51