How did the M/C swap go?????
IIRC, these are a 20mm (7/8" -ish) seal where later Panteras are a 23mm bore? Might be stamped in the large nut that covers the non-business end of the cylinder!
Our Goose takes a common Fiat rear wheel cylinder seal (3 of them) to refit. They are simple donut seals. It is the larger bore size that is a bugger to find!