Sounds to me like a real good start would be to go out and getcha a set of accurate dial calipers and measure the holes and the shaft... ITs a good tool to have around anyhow. Harbor Freight sells cheapo ones but they seem accurate.

Being that all the parts were supposedly just an abandoned project, I've gotta think that they all came from the same gearbox...

If it turns out your shaft is a different size for sure and won't work, let me know... I've got several M40s in various states of disrepair and/or disassembly, and I probably have a spare one of them, and maybe other stuff as well if you need... But I'd need to know the exact measurements.

One clue besides the Jtype OD to tell if its early or late is the reverse gear... The early ones have a gear whose teeth fall off straight on both sides. The later units have a reverse gear whos teeth sides are angled on one side... effectively making each tooth a little wider, hence less likely to break (though it didn't help in my case)

-Matt