Switchcraft (brown) rigid plastic washer for 3/8” bushing. Perfect for insulating metal bushings from the chassis, use with W-SC-S1029.
This washer, along with W-SC-S1029, is for insulating jacks with metal bushings from the panel. The inside of the panel needs to be insulated as well as the outside of the panel. However - the inner edge of the hole also needs to be insulated.
The shoulder washer's (W-SC-S1029) shoulder passes through the inside of the panel hole, insulating the flat part of the panel and the inner edge. This flat washer is used to insulate the flat part of the panel on the other side. If the panel is thick enough, two shoulder washers can be used. To fully insulate, you would normally use one shoulder washer and one flat washer on the opposite side. See the "Insulation Guide" diagram below for an illustration of this layout.
Tone Design Canada