Ok, So for the Obsession I have its virtually the same dimensions as the Extreme V1. The core is completely round not a septagon. It is also a wee bit longer, as in the ends stick out just a touch more. It is approximately 13-14 mm per side. The diameter is ~23.86 mm same as the Extreme.
I have attached a picture of a V1 and a V2 together...
https://drive.google.com/file/d/1TYpE48 ... sp=sharing
Extreme V2: More or less the same dimensions as a V1 but the sleeve is smooth...my suggestions is to move the pen holder up onto the fat part of the sleeve in the groove, that is common between the V1 and V2. Not sure if that would make it uncomfortable to use...my rough calculation shows the pen holder would need to be shortened by ~11mm or so to use that groove.
The dimension of the groove is: ~ 7mm wide and 49.07mm in diameter
HTH
EDIT: Just read your previous post again, and yeah, what you said about moving it ;^)
And one other thing....my V2 doesnt have a Serial number that I can see (its deeper in the puzzle). And the Obsession doesnt have one at all as best I can tell