I don't have a picture as my 1951 is a rigid frame and they had no brake light from the factory. But looking at the parts manual for the later bikes, the switch mounts to a triangular shaped sheet metal plate which mounts to the lower frame tube with a clip near the pillion foot rest.
If you have a M series 1950's parts manual it will make it clear.
Thanks for pointing me to the parts manual. I see the plate and clips, but am not entirely clear as to how to mount them and not interfere with the brake rod. Also not sure the brake rod and lever on the wheel are properly installed on my bike.
A good picture of the set up on a spring frame would be most welcome.
Regarding the centre stand. I spent years fighting to pull the bike onto the stand by hauling on the handlebars and the saddle area then I tried with my left foot on the stand (standing on left of the bike) and pulling from as far to the rear of the bike as possible. It's still not the easiest but a lot less grunt is required. Works for my plunger B31 and M21.
The centre stand was introduced on the post war B31 and carried on to the plunger variants...It was BSAs first attempt at that type and it shows...The weight distribution is all wrong requiring the bulk of the weight to be lifted when operating the stand. The feet of the stand are also a poor design and the pivot and feet wear quickly...When my rigid B33 is loaded with camping gear and has a lot of weight on the back I'm physically unable to get the bike onto the stand...
Really the centre stand is handy for doing maintenance but I've fitted a side stand to mine for daily use...I should have done it years ago..Ian
It's a 1954 B31 stand and spring (swinging arm) which I had 'lying around'...I made the mounting plate, pivot bolt and some longer studs to attach it to the engine plates...I had to 'tweak' the stand leg slightly to get the lean of the bike just right after I'd mounted it...Ian