"the /12 denotes the stanza length"

Three-line stanzas with AAB lyrics were in several of the Child ballads as published by Child, and in e.g. the slave song "Rain Fall And Wet Becca Lawton," and part of "Old Paint" as Jess Morris (born about 1878) said he learned it from Charley Willis (born about 1847), a black cowboy, before about 1900. The use of AAB lyrics was not particularly common in the U.S. in the 1800s, but common enough that when e.g. someone early-born did "Pretty Polly" as AAB (the ones that are much like Dylan's "Hollis Brown"), realistically it's not clear whether that would involve exposure to blues music or not.

Dr. Peter Muir points out that "I Just Naturally Love That Yellow Man" by Larry Deas and John Wilson is from before 1900 and has the ordinary 12-bar chord progression we associate with the blues, but is not a blues. Willie The Lion Smith remembered Wilson as a "dope addict who played piano when he felt like it."

Chord progressions very similar to 12-bar blues were published by Praetorius and Vallet.