I was convinced the original of "It Must Be Jelly, 'Cause Jam Don't Taste Like That" was older than 1942. It sounds like a product of the hot blues rag bands of the 1930s. I speculated that George Williams, with his New Orleans origin, might have heard the refrain from African-American musicians.

After massive search, still in progress, through Godrich and Dixon's (un-indexed, alas) "Blues and Gospel Records 1902-1942" I turned up the following:

"You Know Jam Don't Shake"
Performed by Frankie "Half Pint" Jaxon,
Decca 7733
Recorded March 5, 1940.

Jaxon, a well-known blues stage entertainer, is a good candidate for crossover to white pop music, and may have been George Williams' source for the three-line refrain used in the Glenn Miller recording. I have not heard the Jaxon recording (have any of you?), but the title is close enough so it has a good chance of being an earlier version.

Now, my search continues. I would not be surprised to find an even earlier version, especially as Jaxon, judging by his recordings, often used others' songs in his act. The stanza may be much older; it is not unlike the sort of bordello entertainment Jelly Roll Morton featured, like "If You Don't Shake You Don't Get No Cake" and so on.

I'll let you know if I come up with more.

Bob