Okay. Just to be sure we're talking about the same thing, here are pics of my 510 threaded stuff to illustrate what I'm talking about:
The 1st pic shows the 6ml collar on the left, you can see how it flares out to the size of a 6ml tank. On the right is the 3.5ml tank collar, which isn't tapered but has a lip at the top to smooth the line to the tank.
2nd pic shows on the left an ego with 3.5ml tank - no collar, then with a collar, and the collar alone.
3rd pic is ego with 6ml tank, at the top no collar, bottom one with collar.
Is that the sort of thing we've all been talking about here?
If so, I'd be in for a 6ml & a 3.5ml for 808 ego threads. If I like my 808 ego I'll buy more batts, so I'd probably want more collars if they were available.