Identifying early master cylinders

This method of identification applies to the clutch master cylinder and the 0.7" bore single line brake master cylinder.
These cylinders originally used a seal arrangement with
one cup seal:

seal_cup

and one ring seal:

seal_ring

Later cylinders used two ring seals.
These can be identified by a groove machined around the cylinder between the reservoir and the mounting foot:

mstrcyl_ident