My bike's oil tank and battery support plate is a home made thing, and the regulator sits next to and just aft of the battery. I thought this was a modification a previous owner had done, until I saw a video today about a 1954 M33, the same year as mine. Now I'm not so sure.
Was this a standard arrangement at some point? I've had a look at the BSA range photos for all the production years and although they don't show that side of the bike, the regulator is always clearly visible in its usual position under the seat. I'm not worried about authenticity, I'm just curious.
Here's the video, with a view of the regulator around the 1'30" mark:
https://www.youtube.com/watch?v=oyXlYWA-SAo&list=LL&index=2 (https://www.youtube.com/watch?v=oyXlYWA-SAo&list=LL&index=2)
Here's the nearside of my M33. (Below) The regulator's cover has been chrome plated so it's clearly visible between the battery and the horn.