You are right about the instrumentation, although I initially want 2 pressure gauges.
The only weather shielding was some black paint, more to make the instrument semi invisible.
The blue wire is indeed a temporary lead to attach the scope.
At this point, cutting the tach open was the only solution, I had to see how it is made, and since it would otherwise be scrap, I might as well see if I could learn something by peeking inside.

The speedo cable. Advised by some armchair specialist on a Facebook Savage group, I purchased a cable For the GT500, and although it mated to the spedometer end, it was both too short and did not mate to the engine end. So I mounted the original cable to the engine, the GT500 cable to the instrument, and zip tied them together where they overlapped under the tank.
Then I cut them both with the angle grinder and cut the outher sheaths another 25mm back. Thus there is 25mm inner cable protruding from each cable.
I now take 125mm lengths of 20mm shrink wrap (with glue), feed them on to the cables, and stuff them as far away from the joint as convenient, so that they don't start to shrink when the soldering takes place. I also feed a brass tube, could be any metal, really, and push it up the cable.
Before all this, clean the inside cable with gasoline, heat it with a torch, blow it free from oil, rub it with emery cloth to a shine, and tin it using methods known by plumbers.
Then join the parts, so that the tinned cable ends are introduced into the copper tube, which is also tinned on the inside.
Finally heat all 3 members and apply a bit more tin to the solder joint.
After this, slide the brass tube over the joint, and shrink the wraps down from each side, so that they overjap each other, the brass tube and each their own cable end.
Hope this helps.