Oh that's a pitty, I was starting to get excited thinking about some caps with a counterbore! Maybe I should email them and ask if it's possible to make them.
I know what you mean about the flange. I've heard people say that it stops the tank from pulling off, but your absolutely right it only stops the carto from getting pulled up through the tank. I find even this is nice though, as when a tank isn't attached to anything, pulling off a drip tip could potentially pull the carto up through the tank. Unlikely, but possible

These IBtanks do a wonderful job of hugging the carto super tight, I think back to the smoktech tanks that are super loose and slide off the carto if you look at it wrong.
The only reason I'm using the flanged cartos is because I fell in love with the Atemas tanks, and bought a ridiculous number of flanged cartos for replacements (And I'm lazy and don't want to bother punching/slotting cartos) But then I finally got some Pluid, which I cannot use in my favorite Artemas tanks, so I got an IBtank because I see you promoting them quite often, figured they must be good! And they are, I especially like the "double seal" on the tanks, where it has the normal o-ring inside the tank, and then the additional flat rubber seal on the end. I have not had any juice leak past the first O-ring (I'd imagine it would be obvious as there is a little free space inside the tank, past the O-ring, but before the secondary seal) so I don't know if it's really needed, but it makes me feel more secure.
I was thinking that secondary seal might also be there simply to protect the edges of the glass from the endcaps, to avoid chipping if someone put too much force into seating the endcaps.
Hey OP, that tank does look really fat. Is that the 25mm version?