It depends what gets the G12H designation, but there are about 20 different known/popular ones from 1966-present.
The History Of Marshall book has about a bazillion in there Txxxx listings with all the variations...you can go blind/nuts reading all the minute changs and details.
Keep in mind anything with a 50 oz ceramic magnet is a G12H variant, including:
Vintage 30's - 444 cone / 55hz
G12H Heritages (80's era)
G12H 25w (55hz 014 and 75hz 003 Pulsonic cones)
G12H30 30w (green, cream, gray, black back plastic covers) 55 & 75hz (444 and 1777 cones)
There there are the Heritage 75hz 30w, Heritage 55hz 30w that came out in 2005.
G12H30 Anniversaries came out in the 90's, IIRC.
Don't get me started on 50hz, 40hz organ models, G12H-50w from the late 70's, etc.
I could go on, but you get the idea, it's nuts (and I really don't have the time, sorry!).
Good luck!