Is the red ground wire in the wiring harness on my 1969 triumph bonneville actually one long single continuous wire mounted at different points along the frame all the way to the positive battery terminal
Yes and no.
Physically, there are numerous individual Red wires, that's why "These mounting points have two wires on a single connector, except at the positive battery post"; if you dismantled the harness, you'd find that each of the "two wires" at any "single connector" links to another connector. Electrically, they're all connected together.
Personally, I don't like the Lucas
"ground wire" implementation, copied by most pattern harness makers;
it's a big loop connecting (nearly) all electrical components together with the ends of the loop at or near battery positive; as the harness ages, if any wire breaks, the components are still connected to battery positive by the other side of the loop; however, once there's a second wire break, the components in between the breaks aren't connected to battery positive any longer.
When I build a harness, the "ground" wires mimic the supply wires - one of each between each component and a junction ("ground wire" junctions in the headlamp shell and under the seat), junctions joined to battery positive or negative as appropriate; if anything fails, the failure is easy to diagnose.
The first mounting point is inside the headlight shell on one of the bolts that mounts the headlight shell to the mounting ear. Then at the base of the zener diode. Then at the horn relay mounting bolt. Then at the base of the rectifier. Then at the oil tank mounting bolt.
Uh-uh, 'fraid you and/or the harness builder made several mistakes.
The connections you've listed are so specific components that "ground" through their mounting have a wire (not
cycle-parts) electrical return path to battery positive:-
. "inside the headlight shell" - Original headlamp shells had a loop rivetted in the bottom of the shell, that took a bullet terminal on the end of a single Red "ground" wire; afaict, this was just for the single-wire pilot bulb holder that "ground[ed]" through its mounting in the headlamp reflector, later superseded by a two-wire holder, one wire then being the "ground";
.. if your harness really has a ring terminal here, that's a pattern modification, possibly because either the original rivetted loop becomes useless due to corrosion that cannot be removed between it and the headlamp shell or many pattern headlamp shells do not have the rivetted loop;
.. the original BPF headlamp bulb 'cap' should have another rivetted loop to take another single Red "ground" wire bullet terminal; if you've replaced the headlamp with one that uses an H4 plug, one of the spade terminals is the bulb "ground" and should be connected to the Red wires;
.. the two idiot lamp holders should have two wires each, one each is the "ground" wire;
.. if the speedo. 'n' tacho. bulb holders have only a single (supply) wire each, the mounting plate will need a "ground" wire connection, as it's electrically-insulated from the rest of the cycle parts by the Metalastik anti-vibe mounting bushes.
. "at the base of the zener diode" - The Zener itself "grounds" through its mounting stud, the nearest practical electrical "ground" is the heatsink mounting bolt to the lower yoke.
. "at the horn relay mounting bolt" - Nope; a relay is a switch; by definition, it cannot "ground" through its mounting. The correct 33188 6RA relay has only three separate terminals - supply (Brown/Blue wire to "C2" terminal), 'low power output' (Brown/Black wire from "W1" terminal to horn button), 'high power output' (usually black wires from "C1" terminal to the horns); the "ground" shown in some wiring diagrams is a Meriden Misprint;
.. however, neither the original '69 twin Clearhooters, the later twin Clearhooters (listed in '70 parts books) nor the eventual HF80's "ground" through their mountings, so a Red "ground wire" in this region is likely for connection to the horns "ground" wires or terminals.
. "at the base of the rectifier" - Like the Zener, the rectifier "grounds" through its central mounting stud.
. "at the oil tank mounting bolt" - Mmmm ... the oil tank is a well-known electrical component ...
One thing you have missed is the Red "ground" wire connection to the engine, usually on one of the cylinder-head/rocker-box/head-steady studs; standard points and spark-plugs "ground" through their mountings so need a Red wire connection to battery positive.
One standard '69 electrical component that doesn't have a "ground" wire connection that should have is the rear lamp. If you have the patience, copy what Lucas
did as standard mid-'71-on - a Red wire parallel to the Brown and Brown/Green wires all the way to a bullet terminal into a loop soldered to the bulb holder. In any event, the Red wire needs connecting at least to the internal 'frame' that mounts the bulb holder and the lens.
Finally here, if the harness has only one "ground wire" on battery positive, consider fitting a fuse holder and 15A fuse here. Reason I suggest this is, because this wire connects the cycle parts to battery positive ("positive ground"), if something metallic (e.g. broken seat pan, loose tool or part, etc.) touches the battery negative terminal
, it makes a short-circuit that won't blow the standard fuse in the Brown/Blue wire, because the short isn't through that fuse.
However, a fuse in the wire to battery positive
would blow, protecting the battery.