Well I am down until an O-ring for my rear master cylinder shows up. I am posting this information for future forum searchers, or if anyone has been through this might have an answer. My parts catalog shows no o-ring for the 1975 f0 rear master cylinder cup, only the f1 and some Goldwings.
I know I have read that some of these came all metal with cup built in, not sure when the switch happened but mine has the nylon cup that presses in with an O-ring in the rear master cylinder body. The part number is 45516-371-003, I have been unable to find it anywhere.
I did find its size is 43.7X2.5. A google search for that size shows a Honda part number 91301-MEL-003 for a CBR1000 cylinder head. Some sites list the size as 44.3x2.5 for the same part number. I ordered 1 from Partzilla listed as the size I need. Will report back how it goes. I think this is the last bug to work out so I can finally put some good miles on her.
Edit: That part number worked perfectly, back on the road! It's weird they re-made such an oddball size o-ring for a totally different purpose that was only used for part of 1975 from what I can tell