I have just bought a N.I.B. OS-FS-60, the first model with the machined head and rockers and I note that there is a hole of about 1/4" diameter at the rear of the cylinder about 3 fins up from the bottom. Can anyone tell me what this is for? I know the second series with the cast head and rockers had a breath nipple lower down, this one doesn't so I hope this is an early type of breather. Please help to put my mind at rest.

I believe you're looking at the hole that the piston pin comes out of when you remove the piston for maintainence.
If you can see the steel sleeve thru that hole, then that's what it's for.

Thanks proptop. I wonder why they didn't do this on the second series. Does anyone know?

Some of the later engines were designed so that when you removed the cylinder liner, there's enough room to get the conrod off the crank. The FS-120, and a few others, though, have the hole.