These are my first vintage OPC cards, so you'll have to excuse my ignorance about the backs. Other than some minor cropping differences, the fronts are typically identical to the Topps version.
In 1973 the OPC was nearly identical to the Topps back. The only differences are due to the bilingual nature of the card. Our Canadian friends don't get the cartoon writing, they get smaller type.
I've now got two of Billy's OPC cards. They're first modern release came out in 1965, but for most seasons they had only the first couple of Topps series. If a player's card was a high Topps number, they probably didn't have an OPC card. For Billy, he's got Cubs cards in the 1965, 1968, 1970, 1971, 1972, 1973, and 1974 sets. You've seen the '73 and '74 cards, and the '70 and '71 are on their way. That just leaves me '65, '68, 'and '72. The hunt continues.