I don't know how Norton batched their numbers, but I used to work on classic Ford Mustangs. For the 1965 and '66 model years, they assigned numbers by factory (F, R,T) body style (07,08,09), and engine (U,T,F,C,D,A,K).
a 1966 Fastback Mustang, built in San Jose, California with a Hi-Po 289 engine would carry serial number 6R09Kxxxxxx. These numbers were assigned in batches, and what would have looked like consecutive numbers actually could have been built months apart (production date was stamped on the door data plate, independent of the serial number). Generally, cars were run in batches, by shift or day, and the relative production rates of the various models were based on sales projections, then modified based on actual orders.
Perhaps Norton batched numbers by model, which would mean that a rush of Interstate orders, for example, would run through their allotment before, say, a batch of Hi-rider orders. Therefore, an Interstate may have a number several hundred, or even thousand, higher than a Hi-rider built in the same month. I know only a few parts separate the various models, but assigning a number to a model in advance helps in forecasting needs from suppliers. The interchangeability of seats, tanks, bars, silencers and sidecovers muddles things thirty-odd years later.
I know my '73 850, with a build date of 4-73, has serial 3030xx. As I understand it, production of the 850 didn't start until April, 1973. I don't think they ran through 3000 bikes in one month. Since my bike came with fiberglass tank and side covers, I suspect it was not originally a Roadster, and a previous owner converted it with '71 or '72 parts. Perhaps the first 1000 numbers were assigned to Roadsters, the second to Interstates, and the third to Hi-Riders, or some similar logic.
