They are the same and the pad attaches by the velcro backing. The 3.5" yellow one in the pic is for the PC and the 2 3/4" black one is for the rotary. Depending on where you get the PC, it usually comes with 5" backing plate.
I bought my pc7424 and i bought a velcro backin plate but it doesnt screw on it has a little nub and my porter cable attachment has a screw on type of mechanism what am i missing?