If you look at 69 up rear backing plates, they have 2 tabs on either side of the wheel cylinder. My understanding is they are there to keep the pistons from popping out until the shoes and springs are installed.
For flat-rate, we'd tap whichever tab (L or R) with a hammer to allow the cyl piston to come outta the bore, check, hone, replace cups/pistons/dust boot, then pry the tab back up to retain pistons.