That is a one piece solid tube. What diameter? I'm not a fan of stacked mods but that is a matter of taste. I prefer parallel mods for massive low ohm builds instead of 1+ ohm builds on a series mod.Yeah, when I first spotted it I thought it might be 25mm. It's not going to be my first choice but you never know. I'm thinking I could use the delrin plug that came with my MCM Underground box to run it single battery, too. Hopefully they'll include an adapter for that.
Edit: your MCM is parallel or series?