Identify my bike

Hi all. I have just bought a Mk2 (I think) 850.
I'm confused about the build date on the id plate. It was a US delivery bike and has the build date of 12/1973. Where I am confused is because the engine number is 311172. This seems to indicate a Mk2a, but I thought they were all 1974 builds.
Can someone please let me know the exact model?
Cheers, Snagger.

Your bike was a 73 build of a 74 season bike, the next model year was always introduced at the factory in Aug after the summer shutdown in the previous year. This meant that come Jan of the next year the vast majority of the bikes in the showroom were that years model.

So in Aug 73 they started building the 74 model year bikes, so by Jan 74 the bikes either in the showroom or on their way were 74,s.

My Mk2a is 310410 and is a 73 build too.

In 70 BSA/Triumph messed this system up big time, the 71 build start was delayed by months until late 70 early 71. By the time the 71,s were on the way to the dealers the selling season in the US was over and bye bye BSA/Triumph Group as the unsold bikes were heavily discounted (and were unliked too but thats another story)

'74 Commando

Just to add another bit to your '74 Commando. Norton built both 850 MkII and MkIIa during the '74 model year. The MkII has the previous reverse cone mufflers (silencers) and a ham can air box. The MkIIa has bean can mufflers, black plastic air box and the left side cover fastens to a revised battery tray with a plastic zsus at the lower corner. Here in California we rarely see the MkIIa.

Thanks so much folks. You have been a huge help!