I am guessing here but I thought the metal backing plates were pop rivited to the insert and maybe dome head sheet metal screws were used to screw them into the body tub. in old school days we just used dome head sheet metal screws through the shell in the tub. the inserts back then had molded in dimples in the face so you hit the proper sheet metal bumps under neath. with the advent of one piece units now your kinda of on your own. you need to find a close up of some actual U13's to see what they did. I've never see an actual un molested u-13 so good luck. maybe you will get some close up pictures.
