I just found my old Beta 57A and Beta 58A test files. I did them using white noise, rather than pink, and they may have been done at a slightly different distance from the monitor than the other tests I've run recently, so I can't put them up against my SM58 plot, but the main purpose here is to demonstrate just how similar the mics are. Beta 57A in black, Beta 58A in red.

The only real differences are up above 7kHz, where the 58A has slightly more treble, but in reality, it's probably just down to the manufacturing tolerances between two different capsules.
It may be that the capsules are tested and the slightly brighter ones fitted in the 58As, with the others going in the 57As, but I just don't know. As I said previously, I did try swapping the grilles over, but that didn't affect the results.
Whereas with an SM57 and SM58, although they use the same capsule, the very different grilles give very different end results (SM57 in red, SM58 in black).

Again, you need to make some allowance for manufacturing tolerances between capsules, but a lot of the differences are still down to the grilles.
So, the Beta 57A and Beta 58A are very similar sounding indeed. Which is why I question why you'd really want to pay more new for a Beta 58A when currently the Beta 57A is £20 less.