I'll try to respond to some of the key points that have been raised in various posts.
1. If a light no longer has a serial label, the easiest way to tell if it has a G2 or G3 circuit is listen to the ignition when you first turn on the light. All G2 lights have a spark gap that you can hear when you press the red button to turn on the light. It is a definitely audible zap/buzz. G3 lights are pretty quite although the formation of the arc is audible, sort of a muffled pop.
2. G1 would refer to what was essentially the original prototype systems from 1988. G2 started with the first "production" Maxa Beam in 1989. So it is very unlikely that you would have a G1 system. The gore valve on the side of the main body was added in early 2001 so that would be something that might tell you if it is earlier or later G2.
3. Maxa Beam CandlePower History since 2000:
G2-20 (2000-2004) Last version of G2 circuitry. 75W High and 6MCP.
G3 (2004-2006) First Version of G3 circuitry. 75W High and 6MCP.
G3-12 & G3-12R (2006-2011) Introduction of 85W and 7.5MCP. G3-12R denotes remote-ports becoming standard and occurred in 2010.
****Enhanced reflectors were introduced in late 2006 as an optional upgrade. The letter "A" was added to the model number, i.e. "410-A".
G3-20 and Later (2011-present) Enhanced reflector becomes standard and output increased to 12MCP.
Hopefully that clears up a few points. Let me know if there are any remaining questions.