As far as I know, motor shafts tend to be hardened steel - (something like silver steel) - and extremely difficult to put a thread on - unless its been annealed pre-hand.
I have seen some motor shafts with threads on them, but, they tend to be of the larger sized motor e.g. 240 volts, something like 3 horse power industrial type motors. I did a quick search on the model that DaveB has extracted the motors from, and, they look like the majority of people who built these models fitted something like the 500/600 style canned motors in them.
So, unless DaveB can come back and verify what motors they are, we are all guessing at the moment.
As far as the MFA olympic belt drive system is concerned, yes it is a 6mm thread on the end of the output shaft.
John