I was just looking through some old hobby magazines and found some interesting information about this issue. Collectors in 1968 were also interested in why the Astros logos were missing from the cards. While this article doesn't answer the question entirely, it does point pretty hard at the Astros themselves as being the culprit. It wasn't long after this that all the teams banded together and forced Topps to pay them for the right to show their logos.
