With 79s, I was not aware that a "factory" set was issued, so cards coming from a factory set may not explain what you have seen. While there have been observed differences in vending cards from the early 70s (1971 specifically cards measuring short l/r to fit into vending boxes) I am not aware of the differences you describe occurring in other vending boxes from the 70s .
Through out the years, Topps has used various quality sheets to print their cards on (1966 6th series, 69s, etc). Perhaps Topps used a slightly thicker stock than normal on some of the cards you have seen.
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