A classic basic line hiding supportive underlayment.
Adjustable straps, center seam onback with subtle "v", underwire, full bottom coverage in a more 50's style with a eyelet hook bottom.
 
14" unstretched, 17" comfortably stretched.
Best guess C & 1/2 to D & 1/2 cup sized.
 
Era unknown.
A.E. 1960's