This specific module is just a faceplate, it is used to hide electronics from view and adds some stiffness to the module - this keeps the PCB from flexing under intense gameplay. For the main module, as well as adapters and other alternatives - see C64 JoyKEY for a complete listing.

The A-series of faceplates have a wide cutout that will make it suitable for all Cherry MX-compatible keycaps, the B-series have a narrower cutout that are only suitable for keycaps that sit taller in the key switches or do not descend past the base of them. If you hare in doubt which type you have, then you should probably get the A-series faceplate (or both if you want to - it is hard to stop ordering keycaps once you've started). The number behind is just a way of separating various designs that are printed on them.

This design is in the B-series, they will only fit the taller keycaps that don't descend past the base of the switch. The PCB has some simple writing on it that is a simple tribute to the Commodore 64, as such I feel it would look at it's best with a blue PCB.