the G is just a model designation, started in the '70's as a 540, then, 540A, 540B, 540C, 540D, 540E, skipped F, 540 G, 540 GII starting in '99 i believe, 540 GIII '02 if memory serves me.
8-10 trees an hour is a long running average, cutting trees, like that walnut and white oak you may...