Its an aluminum Ford SVO master, I suspect it was original equipment on some SVO Mustang. Some day I may take it apart, but haven't yet. So I haven't had an opportunity to measure the bore size. We've discussed this master in past threads, you may try a search. I can't remember if we've ever determined a part number. I'm getting to that age where my memory doesn't serve me as well as it once did. If I had to place a wager, I'd bet that the brakes & master were originally sourced from Dennis Quella, you might give him a call. Or give Scott Bell (SACC) a call ... he'll have a recommendation for you.
The brakes are at least 20 years old, the master may not even be available now. Frankly I think anything close to the OEM size will be fine. If you can't find an off the shelf master, why not use a Wilwood master? They make a nice one.
Sorry I couldn't be more helpful.