Thanks guys for all thoughts.
My bike was first registered in UK in 1974, and I got it in 1978. I can't remember how the switches came, but I just happen to have a logbook I kept at the time. This shows that I bought a H'lamp On/Off switch the month after I bought it for £4.64. The following year I bought a LH handlebar switch assy, an ignition switch and 2 footrests for the huge sum of £18! So my LH switch is not original.
Based on your comments, I have done some further research and here's what I found.
The LH switch I have was fitted to CB750 K3(1973)/K4(1974) USA models and CB550 K0 (1975) USA - Source CMSNL website. Now to respond to some specific comments:
Not much use but the Br/B from your LHS control may well be for an indicator buzzer. Correct, Phil
Your switches should look like this. The left hand as a separate switch from the main for high beam. I agree.
This is a very rare bike in the UK, not that many left. Really?
I guess that based on what I said above, I started out with that. When replacing the h'bar switch in '79 I guess rarity/originality wasn't an issue and I took what the dealer offered by way of replacement; so my separate switches were replaced by a combined one. The other sad thing is how my original RHS came back from powder coating.
Earlier harness K0-K2 have round connector to ign switch, later rectangular. The original harness did indeed have a round connector; it was replaced by a square one when h'bar switch was replaced. K2 has different electrical setup on battery holder, fuse holder included. I'm not sure what you mean; there is only one 15A fuse in a square rubber holder.
You need to identify that Br/B from the LHS. It must do something. I reckon it is the buzzer, but since this isn't present on this model, it is redundant.Has it been joined. There should be power to that switch to use the original H/L, unless it has been modded.You mean on the RHS? Yes, that's what I thought too.
Does your bike have a separate 3W pilot bulb in the headlight a la UK? Yes
If so you could use the low beam position on the RHS to power that (via W) and then use high beam (blue) from the LHS and connect to Br/R on the LHS to power low (W), high (B) from there. I'll look at that.
Br/W is rear lights.
Important to check that all this is protected by a fuse. Not 100% familiar with 750, but early 500s just had one main 15A fuse. Later 550s had separate fuses for lights. Same here
Is that additional HI-LO switch really needed?
I have seen it for sale, eBay as well ad DSS.
Switching Hi-LO is possible on right side, right?
I have a K2 project ongoing and had thoughts to add that switch. I have new aftermarket K2 switches from Z1-parts.
I have seen K2 bikes without that switch.
If it was mandatory on K2, my bike will get it.
Fun with the off button for blinker buzzer.
I think it is impossible to be certain about any of this. Honda seem to have been fairly cavalier in what they put on different models - whatever was to hand seems to have been the rule. The separate Hi/Lo switch may have been there on the earlier UK K2 models but not the later ones. Mine is 1974, Fr No 2086***, and based on what I said earlier, may well have had a separate switch. What I can say for sure, is that the buzzer was never there.
United Kingdom K2 switches are different than USA.
P is off
L is side light
H is headlight
And then on the left hand handlebar you have the separate switch for HI beam 
P is off, but for me L connects W and Br/W (and on the WD, but not for me since the wire is absent, Bk). H does the same thing for B, Br/W and Bk (if present)
Surely the Uk wiring should be like mine: the separate on/off switch on my left bars switches ALL lights on/off. No, not on mine.
The switch on the RHS marked P L H then switches P (Pilot), L (Low beam), H (High beam) That's how my switch was marked.
Makes sense.
Phil, do you have a WD to back this up?
The "UK" diagram I have, shows that the pilot/parking/side light is only powered when the ignition switch is in position II. It also shows the RHS to be Off/Lo/Hi, with an intermediate position that has both Hi and Lo on at the same time. I have verified that this indeed is possible, but not something you'd want to rely on achieving at 70MPH on a twisty road on a dark night.
The last pic shows my combo LHS switch with a used one I picked up, showing the buzzer "silencer" button.
