The differences are to do with the valves or valve springs I belive.
Nothing external to determine it though except the 'G' codings... And still not many people who they relate.
G1 - 6speed 1998-1999
G2 - auto 1998-1999
G3 - 6speed 2000-2001
G4 - auto 2000-2001
G5 - 6speed 2002+
G6 - auto 2002+
That table might be wrong but its something like that, sorta relates to your gearbox and the year of production