In the manner of the link that Marc has given.. some recent/current listings that illustrate this topic..

This listing and this listing have the P-adapters visibly fitted (you can see the four screw heads and the two L-brackets.

I think that this listing is one of the third-party backs that doesn't require the P-adapter (can someone confirm?) - incidentally, I like the "remove before flight" tag on the darkslide, that's something I might add as there are no interlocks with a Polaroid back.