Plastic replacement head strap buckle for the Avon M50/FM50/C50 series gas masks.  This part attaches to the rubber tab of the mask and snaps in place with the keeper that is included.  The head strap then threads through the slots and allows adjustment.  New/never issued.  This listing is for one buckle and keeper, specify top or bottom buckle in the notes section of your order.

Top:  has hinged section and is entirely rectangular in shape.  Seen in first photo below.

Bottom:  easily distinguished by one rounded end.  Seen in second photo below.