I have a 1979 V50 and fitted another handlebar to it. When I removed the old ones I took off the switches and on one side a 2cm black plastic tube was on the bar I presume to be insulation but cant remember which side it was on .Similarly on the other side another tube appeared rather more stiff with a rigid piece across it which looks as if it is intended to separate two parts of a switch. Also I now find that the twistgrip is slipping round the bar. I could do to see a diagram showing where these items should be as there is nothing in the manual which came with the bike. There’s also two knurled screws in the throttle body the larger appears to be a there to control the throttle return - a crude cruise control - and a second smaller screw with a spring on it that I cant see what its function is only that it is hard to turn in or out!
Have you got the lego style switches? Mine has them and there’s no bits of tube. Could be a bit of a bodge because they weren’t gripping the bars due to damage or wear.
The original ‘lego’ types switchgear has a short tube inside which acts as a form of insulation but most have been lost over the years. They are not absolutely essential but if you have them i would use them.
To stop the Twistgrip slipping round the bar, there is a very small Grub Screw underneath which needs to be tightened so that it digs into the Handbar. Do this after tightening the 2 housing screws!
The two knurled knobs, also underneath, the one at the rear is a Throttle stop so that you are not overloading the Cables at full throttle. The one in the middle is NOT a cruise control. It is meant just as form of adjustment for the Throttle tube to give a nice smooth action and also to help keep the Cables in place.
Many thanks Steve and Nik - I’ll get back to it today and hopefully have it up and running soon!
To add to Nik’s reply: the black tube will only fit in one side of the switch. There should be a white tube in the other side which is around a mm shorter. As it’s not there, find your switch position and wrap a turn or two of electrical tape around the handlebars in that position. Make sure it only occupies that part of the switch housing; if it overlaps into the centre ridge you stand a chance of stressing the plastic because the shells won’t close correctly. When replacing the switch, tighten up the two main screws until the switch halves are completely together and no further. If you overtighten you can a) distort the housing and/or b) strip the threads. The small grubscrew is to stop the switch housing from rotating.
If it’s like the Tomoselli one on my Le Mans, the friction screw enables taking the hand off for making a hand signal and not losing throttle position 
(like many old bikes had) - the second screw defines maximum rotation of the twistgrip, i.e. a stop for full throttle position - at least it did on mine. I think both were ultimately removed as being more trouble than they were worth.