I have two sets of them, one from a full Game Box set. They also came as pack inserts. Have not seen proofs before but surely there were some and I know the 69 Supers, which also have rounded corners in final form can be found in square poof form. Same with 74 Deckles which can be found as undeckled proofs, or undeckled finals.
Many proofs come from Topps Vault and if so they usually include a certificate of authority. Here you have a grading company saying it is authentic, and it looks good, but the seller has one ebay transaction, which at least raises a flag. If it were a more known seller I believe it might go for a nice premium over the normal game card because there are a lot of Aaron followers. With this seller, some buyers might be leery and hold back. Neat item.
Here are a couple of odd ball game cards I have picked up
