It's more than likely a 5/8" bore if it came off of a dual disk front end. It also has a casting boss on the front of it for a different hose location. All of this I assume you already knew, or suspected. It looks an awful lot like a Yamaha XS1000 MC to me. Take it apart and see what's inside.
1981 Kawasaki Kz1000K1
Located in the Saint Louis, Missouri Area.
I don't remember what bike it came off of. I got it when I was wrenching at the cycle salvage in Indio. That would be in 88. There is one that looks lie this one on eBay for a GPZ 1100. It's junk but if it is the same a kit is available. I'll have to open it up again and measure the bore and compare parts.
Not knowing what the current one is, I'd buy a used one that you are more certain about, and rebuild it. For added information, I checked out Kawasaki's Diagrams, and the GPz550 uses the same front master with internals on the 82-83 H Model as the 84-85 A Model. Maybe a good time to switch to a remote reservoir master from a newer model?