I love little items like this that show remnants of the production process. This one shows that the process was fully along as Christmas approached in 1975. I've seen other mid 70's Topps proofs from various sports and non-sorts sets stamped like this but the one above is really colorful. However, you can plainly see that the sheet information along the right edge is incorrect as it states "1975 Baseball". Weird, right?
Other things can appear on uncut strips and sheets. Witness this star driven upper right corner strip from 1977:
"Slits" in Toppspeak are, in this instance, representative of the two 132 card sides of a 264 master.
Different stars are on display on this 1977 strip: