Green’s Dictionary of Slang

B & S n.

(Aus.) a bachelor and spinsters’ ball.

[Aus]Sydney Morn. Herald 19 May 12: Preparations are in full swing for the annual Trangie B & S, short (for you sheltered city folk) for Bachelors’ and Spinsters’ Ball [Simes:DLSS].
[Aus]Sydney Morn. Herald 14 Jan. 7/1: The B & S balls are a dying breed [Simes:DLSS].