In his excellent book on formula jokes, Jokes, Paul Dickson comments that What's the Difference? jokes, a blend of conundrum and pun, can be found in very old joke books. Some of the examples below are homonymic in nature; others reverse the first and last parts of the name; yet others are spoonergrams.
"The Apple-Sauce Chronicles,"
Word Ways: Vol. 17
, Article 10.
Available at: http://digitalcommons.butler.edu/wordways/vol17/iss4/10