[Edisla]

I don't have one of those models, but looking at the schematic, one end of (8 ) is clearly connected to ground while (8A) is ungrounded. So it makes sense that (8A) requires a non-conductive washer under the screw. The screw and its bushing are grounded. If the screw (or a metal washer under it) touches the plate of the trimmer, it grounds it. So, yes...a non-metallic (plastic) washer is required here.

Exactly.

I'm guessing at some point the original washer deteriorated and someone put in that bandage looking material to keep it from shorting.