those look like regular ol gm 10 bolt backing plates (obviously) never noticed a difference before maybe some kinda change between years.
since no one replied i went out in my pile-o-parts to find some of these backing plates, i know i have them but no clue where they went.
but i did find these and they dont seem to have the hump
came off a 1977 8 lug dana 44. i always liked these better as they had a good solid heavy backing plate not the shet metal ones gm used later on. dont know how rare these are to find i just bought the entire axle for like 50 bucks last year for a project im working on
i wonder if you can just start googling different years followed by "10bolt backing plates" and see if any match the ones you want.