to give an idea of age, you generally find greenbacks up to '73, then creambacks from '73, greybacks from '75, then blackbacks from '77.
the only difference is the colour of the magnet cover with the G12Ms, however the G12H's changed to a different voice coil when the blackbacks were bought in and they are MUCH brighter and unforgiving - a 'crisp' sound that suits more gain than classic rock.