Active Life Keyhole Back Pullover Shirt - S/M

This shirt is in great condition and it's got a large keyhole in the back and thumb slots in the sleeve cuff. The weight/thickness of this is medium. Almost all the tags were cut out of the shirt so I'm unsure of the size. The chest flat measures 18" = 36" around making it either a S or M US women's size. See measurements below. Measurements: under arm to under arm - 18" shoulder to bottom hem -25" sleeve - 21.5" All measurements are approximate.