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.