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Many times there are items found in sewing machine drawers that don't have anything to do with sewing. However, according to Webster's patent 116378 it is an "improvement in button-hole guide-plates." According to the patent, there was another piece to fit on the other side of fabric, but it seems to indicate that they could be used separately.
Based on what I believe the size of the the BH guide plates, I'm wondering if the other item may be a belt buckle of some sort. It reminds me of the clamps used on adjustable web belting. I've also seen it used on elastic belting. The opposing end of the belt would have a hook to fit in the hole. Just a guess.
Janey - Neat people never make the exciting discoveries I do.