This is my interpretation - subject to change
Gen 1 - single mode light, knurled tailcap, no lanyard hole, serial numbers start with 20 or 2T, 30 for the 3AA models
Gen 2 - Multi Mode light ( Hi, Low, Strobe, SOS ), Rebel LED, knurled tailcap, no lanyard hole. serial numbers start with 2R
( I got one that starts with 20, I think Maglite was using up the bodies they already produced )
Gen 2A, or Gen 3, most places just call it Gen 2. Multi Mode ( Hi, Low, Strobe, SOS ) old style tailcap with lanyard hole, these also have a slightly shorter body. Probably the best is to call them Gen 2 with a lanyard tailcap.
Also take note, Maglite uses the same model number for all 3 styles for the same color, in the same packaging.
The Gen 2 lights are nice little lights for the money - around $20 or so..