From what I have seen, there are quite a few varying manufacturers of those, and there are some differences in the height of the heads.
For example, see some of the notes on the product pages for the various brands of Vivi Nova, the Vision 2.0, and Thor heads here:
Coil Heads
I've got a Vision Vivi Nova, and the Vision 2.0 (not sure if those are really Vision) heads also work in it, but are slightly tighter to the top cap.