Thank you for the info, Erik. I was able to find an old Brown & Sharpe 630
thread gauge and the bolt is (as you guessed) the common 28 thread/inch.

So, to reiterate - I am looking for a Combination Square Lock Bolt. This thing:

https://groups.io/g/oldtools/photo/270151/3345582?p=Created%2C%2C%2C20%2C2%2C0%2
C0

If anyone has one that they’d be willing to sell for a couple of bucks, please
let me know.

Thanks,
Dan 

-- 
I'm a simple man. I like pretty dark-haired women and breakfast food.