dont take this as gospel as i'm not an expert on GT's. but from what i do know I think it goes something like this
S1 = Pro Series or Series One or something like that. I know M1 = Mach One
G = Month starting A for January. So this is July
H = Year starting A for 81. So this is 1988
and the numbers are the lot no for that production run.
Hope this helps but get one of the GT guys to confirm as i'm not 100%
Marc