In my opinion, any full matchbook over 40 years old should have a rusty staple. There are dealers that reinstall matches in genuine old covers because collectors like full books. The striker has an oxidizer in the compound to help ignite the matchhead; that's guaranteed to eventually rust an old steel staple.

You should also check suspect matchbooks for extra holes where the new staple didn't exactly enter the same holes as the original. Also, remember that the match makers were required to use narrower staples during WWII to save steel needed for the war effort. Most made in the 40's should have these mini-staples, and many made 20 years after the war continued to do so. After all, they were a tiny bit cheaper and worked just fine. .
.