I think you may be right on the curtain weight. Where I found it may have had a door curtain hanging.I think it's a lead weight sewn into the corner of a curtain to help it to hang straight. It looks the same size as those I've found in curtains, and indeed used when making new ones. You can buy new ones with two holes to stitch them in, like buttons, but all the old ones I've found have just been stitched into a pocket in the structure of the curtain fabric