10 X 1.75 rear drums on 7.25 rear vs. 8 3/4

Did Mopar ever actually use the same 10 X 1 3/4" rear drum brakes on a 7 1/4" rear end as on the 8 3/4" rear end? Here's the deal.....I just yanked out my old wimpy 7 1/4" rear end and installed an 8 3/4".....I installed the correct (and all rebuilt) backing plates from an A-body. I thought I had some 10 X 1 3/4" drums laying around, but apparently I don't. So, now I'm thinking I'm officially screwed....until...

So then I look over at the 7 1/4" rear end....and start measuring.....and find that IT has 10 X 1 3/4" rear brakes on it!!! Is this some kind of option??....my '68 Dart is a GT.....and it appears to be super untouched and original, so I'm fairly sure the brakes are too.

It seems like all the other 7 1/4" rear ends I've accumulated and played with have all had 9" drums on them.....any experts or somewhat knowledgeable folk have some info??......or wish to chime in??? Would those backing plates on the 7 1/4" rear be worth anything?