Henri, motorcycle frame and engine numbers were always in the same position. I assume they were consistent for the sidecar frames as well but as the B4s were finally assembled in the former racing department, it may have been less strict for the sidecar frames. Website position has mostly delivered the number . If it is not there you will have to look elswhere. I would start with scrutiny of the entire front tube. If there is no number to be found, history is blank. Norton will not be giving any answers anymore.:-)
Well chaps, it's not that easy! the number under the spring is the sidecar number, before it was "married" to the bike, so more an internal factory number, like was used on 16H frames, before they got their frame number.
The number that was put in the sidecar frame, after it was attached to the bike was as pictured on the website here, the front attaching lug, sometimes several numbers can be found there, it was matching the frame number of the bike, when it left the factory.
Then there is the sidecar bodynumber plate, and that was inside the locker, on a stamped metal plate, I believe this was the same number as on the frame under the spring, but have no evidence, had some of these plates made for various B4 owners last year, look very nice, will post pictures later.