Hello, the ones I have seen are usually stamped on the petrol tank front mount lug on the frame downtube. It is stamped on the boss of the lug that the frame downtube passes through, not the actual tank support arms. I have scraped paint off before and not seen the number, and then scraped some more before I have uncovered it.
Thanks and regards, Malcolm.
The mystery is solved if you have a look at the 16H frame and engine number page http://www.wdnorton.nl/Frame_and_Engine_number_page_16H.htm.
There is shown where the only valid Norton frame number can be found.
Any numbers found on other places are either casting numbers or the mysterious additions of either civil servants, military guys or previous owners fitting the bike to the paperwork instead of the other way around.