This design is typically known as a 1.25" celluloid button. First time we've seen this large 2-1/8" cello whose back is covered in tin with a pair of 1-1/8" cuts at the center so that that rectangular shape of metal is slightly raised which would have allowed something thin to be inserted through it, most likely some sort of fabric attachment. Celluloid has couple of extremely tiny raised bumps and some light general surface wear just showing a bit in reflected light. Exc. and displays Mint.