Is there difference between MC for power vs manual?

So is it possible that the M80266, that states for Rear Discs, does that mean I am missing the residual valve I should have since my rear is still drums??? I thought the M80266 was on the interchange list, but ??? When I type in that part in Rockauto, it brings up all the other Part numbers that I listed in my first post. Surely they are all the same, right? Also I have installed an adj proportioning valve to the rear, and I have it wide open and it still does not lock up.