[ALR-bishop]

For most early years Topps did front proof runs in which the cards are blank on the back. I am not sure how easy it is to tell a proof from a blank back printing error during the regular card runs. Some proofs are obvious, like the Maris 67 as a Yankee ,the 77 Jackson as an Oriole, or the 84 "Encased Head In The Box" cards, because of front differences from the regular issued cards.

In 68 there were also the separate Milton Bradley game cards. They look the same on the front but have yellowish backs. I think Perry was in that set. Not sure if they did separate proof cards for that issue

But proof or error, I am a variations collector in general and there is not a great market for blank back or even wrong back cards. Even proof cards do not get a lot of attention unless there is a front difference to the regular card like those mentioned above. Perry is a name player and a Perry collector might be interested but it might bring less than a regular Perry card in top condition