This Sony O-Ring is for the underwater housing MMPK-THG. The housing MMPK-THG is for the Sony DSC-T700, and T77 digital cameras. The o-ring resembling a thick rubber band is designed to establish a watertight seal when going underwater.
Manufactured to precision tolerances required to hold a pressure seal on your housing and made of elastomeric material, it provides the means by which a watertight seal is formed.