Rear Brake pressure regulator valve as found on various 60's vehicles but used on 67-69 Camaro & Firebird  Disc or Drum FRONT.. but DRUM rear.

Mainly on 12 bolt rear ends.

  This value was originally mounted on the LHS frame rail  of factory A/C BB cars or modesl iwth a 12 bolt rear end..

Also found on 69 Nova