Thanks Guy. After I posted above I've been flicking through Bruce some more and all along the way it seems the seal caused them headaches through the range, lots especially on different things they tried on the various Premier models and also considering and experimenting with leather on the Senior following sone impact issues but returning to the ring. I understand there must be a small volume loss but with a thin cup this should be reasonably minimal. I would not want to mill the original piston as a threaded hole is one thing but altering it irreversibly is another...