Yes, many older boards did not have software or jumper control of the system bus speed... to change the speed, you changed the oscillator.

It does not have to be an exact speed... if you put in a 49.x mhz oscillator, the CPU will run at 49.x mhz, simple as that. And lower speed oscillators will also work, however, the ISA/EISA speed was usually set as a fraction of the system bus speed, and not all boards had lower (1:3, 2:5, 1:2, etc.) dividers, so a slower oscillator could leave you with a sub-optimal ISA/EISA speed.

is it possibly a 486Dx2-50 ?

Nope, it's a DX-50 😀
I just picked up a 40MHz crystal to play with, it's not much better but the place I looked only had it and 66MHz.

I also think I need to check the ram and cache speeds to see if they'd even run with a 50MHz FSB.

For 50mhz, you can probably get away with 70ns RAM and 20ns cache (and possibly a 15ns TAG chip, though there's a fair chance it'll work OK with a 20ns one).

But anything slower than 60ns RAM and 15ns cache will probably require that you add wait states in the BIOS, which will pretty well offset the gains of the faster clock speed... so it's possible you could find yourself better off leaving it at 40mhz.

From what I can tell it has 20ns cache and 15ns tag chips, but the memory modules are not marked with their speed.
I guess I'll leave it at 40 for now, at least until I manage to find one of those 5v->3v interposers...