Well, just looking at it in the pic you posted, it looks Starrett. Look closely at the heads for a maker's name. If they are Starrett, they would be the forged, hardened, and ground steel heads, not the cast iron ones. They are expensive, much more so than the cast iron heads, and are finished to high tolerances. So, I would grin instead of worry if the heads turn out to be Starrett. Between the various brands, speaking of older USA made ones here, some/most will interchange components with a good fit, some will not. With the locking screws loose, the heads should still be a fairly tight sliding fit on the blade. I like the fonts used on the older Union stuff. They definitely look vintage.
It is a 'thoroughbred mongrel.' Use it with loving care. Next you need to make the missing scriber to fit the 90/45 degree head... Knurled brass head, hardened steel drill rod shank brought to a sharp point with a sexy curved taper. I made mine longer than the original $tarrett ones, there is quite a bit more depth available for a longer scriber, and it is a lot easier to actually use.
