Hello Dave,
I know that the rotors in the Lucas distributors fitted to the P6B and SD1 were different, so I assumed that the caps may well have been to.. :? I have to admit that I have not looked under an SD1 distributor cap, but given the price differences I thought that there must have been a something in it. If Harvey says that they are the same,..
I do have one of the latter Lucas distributors fitted to a Range Rover, and the contact design inside the cap is quite different, and so is the rotor.
If the cap that you are looking at says that it is suited to the P6B, then you will have no problem.
Ron.