About 20 years ago, I purchased a deck. I do not believe it was bicycle and the markings were were upper center back of the design. They would read 7 S or 4 C in standard type font. I have googled for this extensively with no results. Does anyone know what deck this is?

I am willing to guess it was Ted Lessly's Working Performers Deck?

As I understand it, marked decks that use plain text rather than some sort of code, are known generically as "Readers".
Ted Lesley may have been the first to use this approach, I'm not sure. His name is certainly associated with it. I think he might have used white Letraset characters to mark standard decks.
BUT over the years there have been many factory-printed reader decks. So it's hard to narrow down. I guess the location of the marks might be a clue