We know what you're thinking: It's not very sporting to criticize such a charming token from a bygone era of good taste and wit.
But we must. This advertisement is transcendent in its tastelessness!
It's not merely the over-reaching, making-the-figurative-literal pun—"drawing his own conclusion." (Har har!) No, it's that same nihilistic cynicism that equates living creatures with the food they are to be turned into. It is as though the people of Marsh & Baxter, and all people who would consume their products, are unable to make a distinction between a pig and the substance out of which it is made.
Only psychopaths are afflicted with a similar handicap when it comes to humans. We fear and shun those who see humans as mere objects, as walking piles of flesh. Yet when it comes to pigs, this sentiment is utterly conventional among the ranks of suicidefoodsists.
So here, a shyly smiling hog allows himself to be hitched to a wagon bearing the processed remains of his kin. We are meant to believe that even he finds the joke humorous.
Unlike us, he is a good sport.
(Thanks to Dr. Lemur-Cat et al. for the referral.)
Addendum: For another example of hilarity derived from interpreting figurative language literally, see this old post.