Thanks everyone for your replies, I wasn't expecting that!
I just got the bike home and put away in time for the rain to start. I've only ridden it to the end of the block and back so far... Soon as I get some time I'll get out there and measure my tubes to see what I can figure out...
The previous owner said the frame was English based on the lack of the "F" prefix in the number. He owned the bike for 20 years and he believed it was true. Who knows?
This is my first Norton and the whole bike is an amazing and fascinating mystery to me. There is so much to know!
I've always wanted a Commando, for about 50 years now, lol! My brother had one when I was a kid. Being at 62 years, I decided I didn't have time to start from scratch with a project bike as I would otherwise prefer, but instead to find something already nice for once and enjoy! I'm looking forward to much riding and learning about my new old Norton!