as far as i`m concerned there was (what seemed to be) an overlap in the fitment of the knobs.
As a kid (probably 9 or 10 years old), i saw GT`s come into our shop for services & warrantee jobs on cars that were near brand new, from a whole heap of Leyland car dealers in Qld,,, we did a lot of servicing & warrantee jobs for a lot of the car sales dealers like UK motors, Wedmiers, etc,,, Jack (my dad) was well known to actually "Fix" the crap that the factory did wrong,,, so altho it wasn`t "public Knowledge" it was well known "within" the car-dealer scene here in Brisbane,,, that Dad "fixed" things properly,,, & i`ve personally seen GT`s come through, early & late with both types of knobs,,, i`m sorry but i can`t remember a Date for a change-over, but both knobs were fitted,,, it`s 500million years ago now.
I saved my knob (black) for years & years from one of my own GT`s thinking it was such a funky rare knob,,, never did i expect it to fetch nearly $400 on ebay,,, i believe (& again from memory only) that there were more brown than black... & i "Thought" (back then) that the brown was first,,, then they ran out & made Black,,, then re-introduced the brown,,, but again that`s just the way i remember it,,, no cold hard facts tho... My dad would have been able to tell you more... he knew everything
