I have a copy of the BSA Singles Restoration, BSA Twins Restoration and Norton Twin Restoration books all written Roy Bacon. These books have many period pictures. From what I can see almost all of the time, the strap simply wraps around the frame tube or handlebar securing the cable or wires tightly against the tubing.
Every now and then you will see the straps installed the way I have shown in the pictures below. Especially in the BSA Twins Restoration book there are pictures of the straps being used on the frame down tubes and on the handlebar.
There must have been several different lengths of rubber straps, not just one or two. BSA even used these rubber straps on the top tube of the 'oil in frame' BSA's and the frame tubes which are rather large in diameter. Also BSA even used grey colored straps for the 1971 year model bikes so that they would match the frames.
I have restored my BSA 441VS using this type of rubber strap on the handlebar and they just don't seem to last very long. One trick I have done is to use nylon tie wraps and shrink tubing by shrinking a piece of shrink tubing that is cut to length along the tie wrap. Now when the tie wrap is installed the shrink tubing acts as a chafe protector because bare nylon tie wraps can wear the paint away from the frame. Actually it gives it a neat appearance as well.
Rubber Cable Strap - top view
Rubber Cable Strap - bottom view
Note that the strap is pulled tight so that the cutouts in the sides of the strap are pulled through the hole