RR, 17154 doesn't make sense as a Norton number without the "W" as it would have been around 1926 but according to the factory records was never issued. However, there do seem to be WD frames without the "W" There was a war on after all !
Does your engine have a "W" prefix ? By the way, I'm sure Rob van den Brink would be keen to enter your details on his machine register.
http://home.tiscali.nl/wd16h/
Keith, I will keep my eyes open for early G3L pictures (up to frame number 19299 were girder-forked G3s) so there can't be many older than yours. In the meantime, here is Canadian Archives PA - 206876 (wrongly captioned as a Norton on their website)
Rich