I think I can recall reading about PE accessories some time in the late 70's early 80's. Maybe a bit sketchy, but I believe that at the time, they weren't model specific, rather, they were just a generic collection of small wheels, levers, wheel spokes and such.
I can certainly remember the Model Technologies sets from the mid-80's. I think these were among the first model specific (designed for specific kits) sets, with some of the first instrument panels and consoles to feature "3D" etching to represent depth (eg. raised instrument bezels with recessed instrument faces) I still have a small number of these, which I think were etched in stainless steel. By today's standards, they were pretty crude and basic, but at the time they were pretty awesome.
By the latter half of the 80's we were seeing the first kits to include PE in the box (eg. Trimaster, Gunze Sangyo)