The rubber insulating plug (it is specified by an arrow) is installed in some contact plugs of plugs at manufacturer. When you make repairs of an electrical wiring, the rubber insulating plug needs to be turned before again inserting into the plug case.
PERFORMANCE ORDER
1) get and cut off the old contact plug 1
2) remove the rubber insulating plug from a wire and turn it (on 180 °)
3) examine and replace the plug if it is damaged Prevention
In case of replacement the new plug has to be the same color, as well as old, it will allow to provide necessary tightness between a wire, the plug and the case of the plug.
4) put on the plug a wire so that the corbel (it is specified by an arrow) was from the contact plug 1