It looks like a white O-ring, plus some black ring around that, but I don’t think dorpmuller explained.
Same here:
For example, in a P60 head like that, you could press the front of the Carclo to the lens if you have a pill extender, but there’d be space (like ~6mm) around the Carclo.
EDIT: Also, in the new 26650 triple from CNQ, they have a black ring thingy.