Tool unknown

[Toolmaker51]

I agree it's tooling, likely presented upper surface of work at the angle to drill, mill, or grind.
Hard to say if whether oriented as shown or an angle to that; such as angle plate or rotary table etc. It might been used with location devices to set parts dependably on a certain location as well.
As a bare plate it's 'tooling', used in conjunction with a machine for results.
With clamping, it's a 'fixture', actually holds part securely enough to perform work.
With items to guide tools, such as drill bushings, then it is a 'jig'. Jigs are designed and built with a certain type operation in mind, and usually a particular type of machine.

None of those restrict how you can utilize it, infinitely. While 'drill, mill, grind' are obvious, it could be attached to a lathe [albeit large] face-plate or fixture on centers, very common for work on castings, notoriously challenging to clamp properly.
Guaranteed worth more than it's weight in scrap. Whatever work done in previous shop might be a clue; but they could have acquired and saved it, as we say, just in case...If I tripped across one, I'd take it too.
So, real answer is dependent on someone's need + creativity!