The easiest way to sort this out is to join the AOMCC and ask the club registrar. He will provide you with all the info from the original factory records, including how the bike was set up, what colour, extra's etc were with it and where it was shipped to. All by itself, this is worth the cost.
The longer way is to do the detective work..
The number you gave is correct for a 1949 frame, the engine number should start with DJ - this only tells you it's the right year 350 engine and frame.
If original, the carby number is different for an NH and NG, as is its bore size http://www.barkshire.co.uk/AOMCC/Knowle ... spx?id=127
Look inside the inlet for porting marks NH or rough cast NG.
If it is apart, the flywheel numbers will also point to NH or NG. http://www.barkshire.co.uk/AOMCC/Knowle ... aspx?id=24
A 1949 NH would have a 20" front wheel, the NG a 19"
The cosmetics were different - less chrome on the NG and black paint.
NG's were great bikes and are rarer than NH's, I think due to NG's being badged as NH's during restoration, so if it turns out you have one, IMHO, you are just as well off.
Any or all of these things could have been modified / replaced over the years, depending on the history of the bike.
I hope this helps, Brenton
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.