I think this is called a sash buckle, ladies wore them as belts, and you are missing the other half (the end with the hook) that matches the half you found.These are fairly old and nice finds, good job. Niffler
Sash buckles were very popular during the Victorian era up until the 1920s. Yours looks to be a later model because it was stamped metal instead of molded. Here's what a couple of older pairs look like: