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Over the past 10 or 15 years, my collection has been predominantly pre-war and post-war baseball cards with just a small number of 50s era post cards. Those post cards have been pretty easy to verify (I think every one was used and post marked) and also very low risk, low cost. Im starting to look into a few 1920s era RPPCs and several are unused.

What resources can I use to educate myself on the validity of these items? What tips should I look out for? Ive done a tiny amount of researching on the stamp boxes being guides for dating issuance; but I definitely dont feel Im seasoned enough to jump head first into a purchase that is significant enough $ to really bum me out if it turned out much more modern that presented.