I have an idea that there is a diference between the US understanding of a VIN number and that in the UK. I don't recall the term 'VIN' being used in the UK prior to the introduction of the standardised 17-digit VIN number. Prior to 1981, we simply referred to 'Frame Number' or, in the case of cars etc., 'Chassis Number'.
The situation prior to the 1980s with UK motorcycles was that in theory at least, any number of manufacturers could use the same number sequence. Pre-war, this clearly happened except where model prefixes were used.
It is strange that at some point, Woolwich became lazy and discontinued the practice of the last sixty-plus years and stopped physically stamping the number on the frame. They also stopped using a model code identifier for the frame, although it continued on the engine numbers for a while. This was an invitation to fiddling and theft and I can't understand why the authorities let them get away with it.
The implication is that the number on the headstock plate was indeed the 'frame number'. Presumably in the US they called it the 'VIN' ?
At some point, I wonder if under pressure from the US who already had a 10-digit VIN system, Norton began to stamp a....10 digit ...number on the frame. This number would have had to be unique to Norton and the *850*F prefix would have ensured this.
It is clear from the copy UK vehicle registration applications provided with new motorcyles by Norton (many of which are held by the VMCC), that Nortons regarded the number stamped on the frame as the frame (or VIN) number , where it differed from the engine number as this was the number that they provided to the authorities. Once the 10-digit number was used on the frames, the number stamped on the red plate ceased to be the VIN number and confirmed only that the engine number fitted met US standards (Emission, noise and suppression ?) The number stamped on the frame is there for a reason and it is not simply a factory build identifier.
I know that this system sometimes went wrong, even in the UK and after querying matters with Norton in the early 1980s, my vehicle docs were amended to 'matching' numbers per the red plate and it was only when the frame was blasted that the *850*F1 number turned up again, precisely as the original paperwork. It was quite a job getting that lot sorted out.