GS750 front master cylinder piston and cup replacement

[Stretch203]

Just received a replacement piston and cup set (Tourmax MSB-317) for my 1977 GS750B. I should've stripped the master cylinder first (schoolboy error!) and checked the bore size, as the Tourmax is 14mm diam and mine is 15.7 mm diam ish (piston seal diam). I think previous owner has fitted cast wheels and twin disc set up so the master cylinder is not the standard one but off a later model GS with twin disc setup?
Seem to be struggling finding a piston and cup set for my master cylinder, can someone confirm that the Suzuki 59600-45811 is the 16mm bore version? Also everywhere I seem to look this is discontinued or out of stock! any advice would be welcomed. Thanks

[Nessism]

Unless your bike is a full on "restoration" and you are going for originality, I'd ditch the master and get a new one. 5/8" or 16mm, both will work.

[uk gs nut]

Bantit 1200 masters work well, and have span adjustment