While the original label says that it is a brassiere, it look more like a bust bodice. It dates to C.1913 and is heavily boned. It would have been worn with a straight tubular corset, as those corsets didn't offer anything in the way of bust support as earlier corsets did. Its made out of cotton coutil with Broaderie Anglaise on the bodice and fastens at the front with heavy duty hook 'n eyes. It also has a tab with a loop on it to attach to the studs on the busk (this would anchor the bust bodice to prevent it from riding up). The stud you chose to anchor it to on your busk, would depend on your height, of course.
It measures 38" around the bust and is labeled