That's a great circuit for a voltmeter (called "dipped lights" in the wiring diagram, it is the low watt "city light" in the headlamp assembly, the tail light, and the instrument illumination). This is powered by the ignition switch through Fuse #6 (no relay).
Probably good to check your voltage drop from the battery to where the meter taps in and add that back to the reading. I reckon it should be less than 1.0 volt, but could easily be 0.5v . . .
So great to have @Kiwi_Roy posting (his great ideas and contributions) again. Welcome back, sir!