What's the joke behind this sign?
Sha na na was a "faux retro" group that sang doo wop songs from the 50's (which is why Abe liked them). Bowzer was a popular member of the group (the joke in the show was a goof, however, because Bowzer didn't join the group until after their Woodstock performance in 1969).
Jon "Bowzer" Bauman (born September 14, 1947) is an American musician, best known as a member of the band Sha Na Na, and game show host. Bauman's popular Sha Na Na character, "Bowzer", was a gangly, slender-armed greaser in a muscle shirt.
Conceived by George Leonard, then a graduate student in humanities, Sha Na Na began performing in 1969 at the height of the hippie counterculture, and achieved national fame after playing at the Woodstock Festival, where they preceded Jimi Hendrix. Their 90-second appearance in the Woodstock film brought the group national attention and helped spark a 1950s nostalgia craze that inspired similar groups in North America, as well as the Broadway musical Grease, the feature film American Graffiti and the TV show Happy Days.
Amusingly, Bauman wasn't actually a member of the band when they performed at Woodstock.