I have the mdx bass turd v2. The sleeve for it fits on 21700. It goes in the lower part of the stack, which is bored out to accommodate the sleeve. Basically it is simply to make sure the 2 batteries make contact with each other. I tried without it and it wouldnt fire most of the time and had a noticeable battery rattle. So if this is the sleeve you are referring to, it goes between the batteries.Mmmh still trying to figure out what the adaptor does. It just looks like a sleeve to get a 18650 to fit in the mod. Mabye I’ll just get one and try it out sometime. I have a topside as well.
Many mods do also come with one so that a 18650 will fit a 21700 mod. Same concept. I have a box of them since i use 21700 lol.