This is a production made and used Bio Monitor 'buckle' from Star Trek: The Motion Picture.
This is one of the smaller style that were used with the female uniforms. The item is a vacuformed plastic shell that is painted. This particular example is 'filled' and has two small straps to attach it to the front of a costume and shows signs of use from the set.
More from Star Wars: Episode IV - A New Hope (1977)