1930-1931 Headlight Plug Insulator, 3-wire, Pair

The black plastic inserts that go at the base of the headlamp where the wires butt together.
This is what joins the contacts in your headlight sockets to the wires coming into the headlamp from your lighting wire harness.
The headlamp conduits are what holds these and the wires in place.
The later 1929-31 headlamp buckets had the larger 13/16" I.D. socket holder in the base of the headlamp and used these A-14584-B plugs. U.S.A.