Replacement plastic housing from a pair of 4.1Bs. This is the switch side arm, which the cable connects to, in a matt black colour.

This arm has PCB mounting bosses 51mm apart. If the holes in your PCB are around 25mm apart, then you need a 4.2B Type 1 arm instead (see the related products below).

This part is a 2nd hand and has been removed from a broken pair of GHDs.