Manufacturer's part #02 MFG 015. This is a precision 25mm diameter by 6mm thick 1/4 wave not aluminized mirror. It is polished on one side and ground on the other. Low Expansion Borosilicate Glass (LEBG). Unused. Could be aluminized to make a high reflection mirror or used as a mirror in applications where only a small amount of reflection is wanted such as sun reflection or certain laser applications.