I own a 1974 Mark IIA Interstate however, it was first sold in February of 1975 (I have the original sales receipt for the bike from Tillman's Sport Center, Los Angeles,CA). I am sure it was the standard practice of the motorcycle dealers to put the year model of the bike the year it was sold. As for my motorcycle, I wouldn't be surprised that the dealers got the Mark IIA's at a real cut-rate deal. Apparently Mark IIA's weren't selling very well in Europe so they were dumped into the US market in late 1974 to early 1975. I would think most of the Mark IIA's were licensed as a 1975 model in the US. As a matter of fact I remember my dad during 1975 finding an ad for 'Brand New Norton Commandos' in the Los Angeles Times advertised still in the crate for $1495 (of course after tax, license and dealer prep the total would have been a bit higher). My dad showed that ad to a work associate who purchased one of them. It turned out to be a 1974 850 Mark IIA Roadster painted in the "John Player" colors. I am sure his was licensed as a 1975 as well.
If you want to worry about something, its the VIN number itself. On my motorcycle the VIN number is stamped on the right hand side of the headstock (850F10xxxx). I was concerned about that even though my original sales receipt has that number written on it. Over 20 years ago I talked to Chris Scott the owner of SuperTwins in North Hollywood, CA about the VIN number location and he was adamant that my bike was registered correctly. He explained that a lot of dealers were doing it wrong by using the red certification label number instead of the VIN. However, he felt (and I do too), what difference does it make? We are at the mercy of what the dealer puts on the registration form when it is first sold. Now you can look at it as a little piece of history about the way things used to be done.