Now, I don't know if NBC did this, but many shows from that time (on ABC at least) had a billboard (still photo with the name of the series on it) come on right after the final fade out with one of the leads saying something like "This is William Shatner; stay tuned for some exciting scenes from next week's episode."
Could that be what you are referring to? Your local UHF station probably ran the 16mm syndication films uncut, and those would have the bumpers and billboards attached. I have a 16mm episode that has mid-episode bumpers and the sponsor cards, but the end credits and previews are missing, so I can't check. But this would make sense, since the previews seem to be unedited aside from that "Next Voyage" clip. The aduio was untouched (except for The Menagerie).
I think what you and judexavier describe is exactly what it was! Thanks for jarring my memory!
Jude -- you saw it on the TWOK DVD?