Making the bolt
If you choose to use a shoulder, one can be constructed out of brass bar - simply drill a hole in some brass bar that is the width of the shoulder you desire, cut it to length so it will work as a shoulder, with maybe a final filing to shape. I used a 10-24 square steel nut, soldered onto the brass rod just below the brass nut:
Anyway, I thread a nut onto the end of some threaded brass rod, and cut it off at just over 1" in length:
Another option is to buy the threaded brass rod in 1" lengths, then there is no need to cut them...
I then silver-solder the nut onto the brass threaded rod using a torch, some flux, and some silver-solder:
The threaded rod sticks a little past the nut, so it's back to the sander to level it off with the nut, using an additional nut on the same threaded rod assembly still chucked up in the drill. If you did it all right, it should run true:
Then its back to the de-burring and polishing wheel to clean up the face of the nut - you can see some assembled bolts in this mess:
At this point, the split nuts and bolts are complete and ready for installation into your new saw. If you drill the hole to the proper depth, little filing should be necessary - you will need to file and polish some on the nut side of the saw for sure, however - as the bolt will undoubtedly protrude beyond the nut somewhat.
Here's the face of the bolts, installed in the same saw as the photo above:
When assembling, a little Loctite Threadlocker will also help to hold the nut in place, but I've not found it necessary.
While these were made for new saws, there's no reason this same technique can't be adapted to make replacement nuts and bolt for vintage screws.
Another fun little project!