Thanks for the explanation : true, forgot about that malarky of "matching" backs indeed, once you have a P45+ of X mount, you can mount on every X camera of that that mount !Sorry if this was unclear. I mean that within the mount you can change to any body (of that mount). With other brands the back and body are locked arbitrarily together and you cannot use your back with another body (as comes in hand for backups, replacements, or upgrading to a more recent body).
With a Contax mount back any Contax 645 body ever made will work with any Phase One Contax mount back. Likewise any M mount back will work with any M mount body.
An H3D-39 digital back will not work on an H4 body, nor even another H3D body unless they have been "matched" (unlocked) at the factory.
Still, the P45+ is a great mount with features that were ground breaking when it was released, make it a very actual and updated mount today.
Best,