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
Britt
Below: Old Style A19