This item belonged to my father, who enjoyed making guitars in his lifetime. It's a plate used to evenly apply pressure and prevent slippage when gluing the X-bracing bridge plate. I believe it's made of resin or something similar. Even if glue gets on it, it comes off easily. My father always emphasized that the bridge plate not only needs to be firmly attached to the top wood but also in close contact with the high and low-frequency bracing to alter sound transmission. It's useful for preventing slight shifts when directly clamping the bridge plate, as the clamping force (which isn't just downward but also sideways) can cause it to move. It's also helpful when attaching the bridge itself, allowing clamping without interfering with the bracing. There are signs of use and old scratches. Please consider this a used item before purchasing. Guitar Repair Handmade Guitar Woodworking DIY Martin Gibson Merrill Santa Cruz Vintage Guitar