Don't know the exact but since it is a "75th Anniversary" buy back it isn't worth as much as a regular '71 with an auto would be.
I think its a cool idea that topps is using these old cards to get sigs of older players still around, but the stamping thing deflates the value as a "vintage" card and turns it into a modern autograph card.
I agree I think they should put a separate card in the pack and say it's authentic and it's for the 75 years of trading thank you for your feedback mate.