I have some of those 79/78 cards too. I think Topps used whatever cardboard was around as unfinished product for press adjustment or to produce other products.
I also have baseball fronts with Mork and Mindy sticker backs - not actually stickers but the sticker image.
And a Bazooka bubble gum box with 1978 Topps baseball backs inside. Another has black hole card backs inside.
Steve B
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