| Diagram | Fig. 17 — NACELLE HEADLAMP AND SWITCHES · ref 20 |
| Subsystem | electrical |
| Listed in parts books | 1960–1974 |
Adaptor c/w leads (99-1003)
Lucas headlamp bulb holder and adaptor lead (554602 / 99-1003) that carries the high and low beam circuits to the bulb inside the headlamp shell.
554602 is the Lucas headlamp bulb holder and adaptor lead assembly that sits inside the headlamp shell, wired to carry current from the harness to the bulb itself. It shows up across a wide run of catalog figures covering nacelle headlamps, headlamp and flasher assemblies, and the wiring harness pages for both magneto and battery ignition setups, so it's a part that turns up on a lot of different Triumph twins over the years rather than one narrow application.
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This is the socket that the bulb actually plugs into, carrying both the high beam and low beam circuits, and it's also known under the number 99-1003 in some parts books. If your headlamp has gone dim, flickers, or one filament won't light, a tired or corroded bulb holder is one of the usual suspects before you start chasing the harness or the switches.
Given how many different figure titles this part appears under, from early nacelle headlamp setups to later separate headlamp and flasher lamp arrangements, it's worth double-checking against your own parts book or bringing in the old one for comparison before you order, just so you know you're getting the right lead configuration for your bike's wiring.
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