Congrats on finding both halves of that Victoian era sash buckle. I've found a few halves before but never both pieces. Definitely looks like a coin you have. Looks pretty smoked. If it were me I'd try to scrape the verdigris off of it with my fingernail or something similar and see if anything jumps out at you detail wise just for ID purposes. I wouldn't hold my breath though, as that one looks pretty smooth.