Green’s Dictionary of Slang

the saloon bar of a public house.

1937		J. Curtis You’re in the Racket, Too 267: ‘Balloon?’ asked Len with his shoulder against the door of the saloon bar.
1960		J. Franklyn Dict. of Rhy. Sl.
1969		S.T. Kendall Up the Frog.
1977		D. Powis Signs of Crime 172: Balloon, the The saloon of a public house.
1989		J. Morton Lowspeak.