yeah, looks to be a single 2-2.5mm, def MTL.
I've been running my nuppins with the same coil position as that for a while now, a single coil in the center right at the driptip outlet instead of at the air inlets. works awesome with the nuppins big dual air slots open and hitting the coil from both sides.
I've been running them without the afc ring so all 3 slots open, but then you can still draw softly for flavor that's off the charts, best vape ever, right up there with a chalice when you draw softly, but then you can also fire a little longer and draw a bit harder for clouds on top of clouds, all with the same setup.
not the prettiest things but works good enough to put everything else on the shelf for the moment. I love this single coil at the driptip outlet setup. I really want to make a custom cap for the nuppin that has many small holes all the way around it to try with this setup.