The Non-Sports Encyclopedia by Chris Watson has entries for both Topps and O-Pee-Chee Giant Funny Valentine card issues in 1961. I take it that the Topps version has orangey-red backs, which most collectors term "orange" although I think they're closer to red than to orange. Nonetheless, the words PRINTED IN U.S.A. appear on the backs of the Topps cards while PRINTED IN CANADA appear on the backs of the OPC cards.

Christopher Benjamin states in his Sports Americana Price Guide that these Giant Funny Valentine cards were reprinted in 1967. The 1967 Topps issue was printed a much darker, purer red colour. Both Topps sets were printed on white card stock.

The OPC Giant Funny Valentines were printed on grey card stock though. I have a complete set minus two of the OPC's printed roughly the same colour as the Topps red backs from 1967 only on grey stock which gives the backs a somewhat more dull appearance. But I have also seen OPC Giant Funny Valentines printed a very deep red/purple on grey stock! So then which OPC cards were printed which year?

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A thank you to Dan for posting the pics!

My understanding is that cards 5 and 8 are Topps cards from 1961, cards 6 and 9 are Topps cards from 1967, and cards 17 and 34 are O-Pee-Chees from 1967 (I think). The near set of O-Pee-Chees I have are roughly the same kind of red as numbers 6 and 9 but are printed on grey stock which gives them a duller appearance.