Hi Steve,
The Ford MPC shows the C5ZZ-18813-B as the service replacement for 65 to 68 Mustang. So it is likely not the right one for 6S406 and is the later style. I don't now about the C5ZZ-18813-A, but it is the same part number for the one Bob sent the link. So I think you are after the -A version.
I do have a lead on another -A that I will send you in a PM. You might be able to get a better price.
Cory