Someone on this forum is sure to know, and this information was occasionally included in the manuals.
I have run into similar problems myself. What you can do is try to measure across the thread diameter with an imperial ruler or gauge. This should at least tell you if it is 3/4" or 1/2" or whatever. You will find most of the old shaft threads are either UNC (coarse) or UNF (fine) - but I'm sure there will be exceptions. You may also need to count the number of threads to an inch. Some shafts have specific thread counts (TPI).
If you get stuck, you could try taking the whole thing to an engineering works or a nut and bolt specialist. They should be able to tell you the exact type of thread and size. Good luck!
