As mentioned in Mark Gruenwald's Origin of the Bullpen Bulletins, differences in publishing schedules meant that the Bullpen pages for any given month were not always the same. Despite his assertion that the first modern MBBP was printed in all Marvel books cover-dated December 1965, it actually appeared "earlier" in some November issues, like Avengers and the then-bi-monthly Uncanny X-Men, while that month's issues of Fantastic Four and Amazing Spider-Man featured a t-shirt ad.
One thing that's interesting and fun - okay, one thing that's interesting - about the Bullpen pages and advertisements are how they were tailored to the comic they appeared in. In Fantastic Four an ad featured the ever-lovin' blue-eyed Thing exhorting readers to go "T-Shirt Wild", while timid teenager Peter Parker was shilling shirts using the very same sale pitch over in Amazing Spider-Man. In the strip to the right, characters from various books (Including a bizarrely warm J. Jonah Jameson!) directly encourage "their fans" to join the Merry Marvel Marching Society, while in the collaged panel below sneering villains do the shillin', press-ganging America's youth into the MMMS while revealing their Canadianess, eh?
Yeah, it was superficial and blatantly commercial, but it helped make the characters more real to the fans, breaking down the fourth wall with mercenary merriment. The artwork ranged from horribly drawn originals to headshots clipped from the comics themselves. With little consideration given to context, a phrase like "Can You Guess What's In This Mysterious Mailing Tube?" is delivered in a myriad of ways depending on the image used in the advertisement; Professor X goes with the classic "dramatically accusatory and cross eyed" approach, The Thing nails bewildered befuddlement, haughty Doctor Strange can't even be bothered to get off the Can of Cthulhu to emote, an all a-flutter Green Goblin seems to really, really, really like the tube, and Hawkeye... well, I still can't get over the Green Goblin.
He's got plans for that mysterious mailing tube (Whose contents were revealed two months later in the January, 1966 Bullpen Page).