Jeff,

Your right, the service manual specifies 520 and 525, but the sizes are different between DID and RK. I would suspect that a 520 is a 520 is a 520, but based on what the service manual says, I would get a 520 in RK and if your getting a DID, get a 525.

You should be able to cross reference the model #'s in the manual to come up with a Regina or other chain of the same dimensions (I would guess?).