the silver canister on the back you mean? And that is just another place to hold the gas, that's comonly called a piggy back do you have another pic of the shock? One where we can aculy see the rest of it?
Im pretty sure at the top of the shock, not the piggy back, theyre should be two square shaped cut outs, and theyres a black tool that fits it that comes with the bike. just spin it. i think thats how u adjust it.