Norton used the harness clip on the right hand fork leg. The dipswitch cable seemed to run down the bars and then inside the fork side plates. To attach the brake cable to the girder blade they used a couple of turns of cloth tape. This was still present on my fork leg and appears in 1940 photos so I replicated it.
Has that BSA not got one clip, incorporated in the harness and then a rubber or metal tie above for the dip switch wiring ?
From my point of view, as a 1940 buff, I think BSA owners have been too unquestioning for too long and have not gone back to original sources with a critical eye. The fact that M20s were in use for so long and were then sold off in post-war WD trim has made it all too easy, but the gene pool has been contaminated by 1950s supersession parts. There has probably never been a post-war WD16H replacement wiring harness which meant that owners had to make their own and transferred the clip over but ex-WD M20 looms minus the fiddly clip were probably available off the shelf until recently.
Was the loom a Lucas part, as per the battery carriers and headlamp assemblies ? It would seem likely. I wonder also if Russell Motors have got a box of the clips with LV7/BC or LV7/LU part numbers and no-one ever asks for them ?