The difference I believe is the position of the internal contact face which means that some G2's don't make contact with the newer A19's. My assessment is based on the G2's and the A19's I have.
I did read that new A19's have been working on some G2's though.
If you're not wanting to use the G2 then there should be no operational difference.
I assume the G2Z is like the G2. However, I am not in a fit state to be trying such combinations for you. If this is of specific interest and importance and nobody else replies, please send me an email.
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Surefire makes an adapter for the G2 to become a G2R longer
than the A19 but with a spacer it could be used instead of the A19 to add a battery.
Topper /ubbthreads/images/graemlins/smile.gif
Stingray, here is a photo (courtesy of Size 15s) that shows how they differ externally.
I have only experienced problems with them on the G2. I bought a few G2s last year and equipped them with A19s for 9V use. I found a couple of old style A19s and a couple of new style A19s. I think one of the old style A19s worked with one of the G2s. But all the rest wouldn't work for the reasons Al mentioned.
Here are a couple of photos showing where I placed a wire "washer" and it fixed the problem, for both old and new style A19s. All of the lights are still in 9V mode and have had no problems in a year. Inelegant, but a good field expedient fix. /ubbthreads/images/graemlins/yellowlaugh.gif