Frame / Engine Numbers Question

Thinking of taking a look at an 850 Commando with the frame/engine # 305548/308170. This suggests to me that the frame was manufactured in mid to late 1974 and the engine was made in early 1975. I understand that all of the Nortons match until around 1974 -- can anybody tell me whether this could be the original engine, or if not, what I should look for when I examine the bike? Thanks!

One more thing -- is it the VIN# (stamped aluminium badge riveted to frame) or the frame # (stamped into frame) that is supposed to match the engine number?

850 numbers

frame/engine # 305548/308170

These two numbers should be the same leaving the factory. 305xxx likely late in the '73 model year and 308xxx is in the '74 model year. The aluminum frame number plate can have either the same number as the engine or the Fxxxxxx number stamped on the frame tube. My '74 JPN had 112259 on the frame plate and 316995 on the engine. California used 112259 for the VIN, but also noted the engine number on the title and registration.

As Commandos are getting older, it's more and more common to get mixed engines and frames. Your numbers aren't that far apart.

Thanks, David! My understanding is that the late 73/early 74 designs do vary tremendously, so I should not have any trouble treating this bike as a Mark II/IIA for maintenance purposes. It would be nice to find a matching numbers bike, of course, but for my first Norton, I would just like a good runner that I can enjoy. I appreciate your help, and I will let you know when I join the Norton brotherhood!

Whoops! That was "do NOT vary tremendously" My bad!

Nomadwarmachine,
You shouldn't have any difficulty with parts or interchangeability between '73 and '74. The major difference is the cylinder head. The '73 engine had 32 mm intake ports and intake manifolds that were 32 mm on either end, while the '74 head had 30 mm intake ports and the manifolds were 30 mm at the head and 32 mm at the carb. Just don't mix and match intake manifolds.