The SureFire AN14 is a Nitrolon extension tube that comes with the rechargable battery kit for the G2's. IIRC, it's 1.5 cells long so if you use some sort of metal spacer (if using a metal spacer, wrap it in electrical tape or something so that it does NOT touch the sides of the battery tube) or dummy cell, you can add another cell into it.
There is no Nitrolon equivalent to an A19 extender or one like Mark A. linked to.
I remember it wasnt that long ago that Lighthoundbow had the SureFire AN14 Nitrolon extension tubes in stock. Drop them a line and see if they can get them in again. I seem to remember they used to have both the black and the yellow.