As has been suggested the black one fits the rigid frame A's including the longstroke Star Twins and the rigid and plunger B series. The red one is off a C10 or C11, the recess on the back is where the brake rod passes. It could be suggested the C10/11's have a better rear brake set up than the A10's, they have a cross over set up but with a rod rather than the cable the A's have.
You shouldnt have too much trouble finding the correct one LJ, they come up on evil bay quite regularly or a swap should produce one.