Well, I think more are popping up because of a decoding of the old Yankees tickets. Many were not dated and just had the game number on them. But they also had the price and a letter on them. Some ticket collectors have cracked the code and written an article about it...
http://www.psacard.com/Articles/Arti...-1933-1947-era
so more may be coming into the hobby as people find out about this. But if, like me, you require an actual date on the ticket it may be harder.