It's always a good idea to have spare O-rings for your underwater camera. Spares are your insurance. This is an O-ring for the Sea & Sea Motor Marine II. It can also be used for the Motor Marine II EX. Huh? You don't know the Motor Marine II? For those people, I'm also listing the Motor Marine II EX. Spare O-rings are hard to come by these days, so I recommend stocking up on them while you have the chance.