There's an earth wire connection to the reflector in the R510 - I use triple armoured cable for the headlights, side, head and earth to each one.
These lights are installed with a Lucas SM3 panel, which is (was) wired to supply two 3W filaments on sidelight position and switch to two 18W (12W originally) filaments on main light position, so they are direct replacements for the 3/18W filament bulbs with no change to the wiring.
I believe the CAV panel, used until Autumn 1926 was arranged to work with two single filament bulbs, putting them in series for sidelights and in parallel for main lights. These LED lamps would not work with the wiring arrangement for the CAV installation.
Apparently, in times gone by, the way out of this was to get an Austin 12/4 CAV panel from the scrapyard (it has a 20-0-20 ammeter) as this is internally wired to support conventional, twin filament bulbs, like the SM3. Last time I checked, my local scrapyard was fresh out of Austin 12/4's