Firstly, I'm no expert on this. If anyone can help me out I'd be pleased to hear from them.
Many articles and books on Nortons skip quickly from O'Donovan breaking records at Brooklands in 1914-15 to the OHV 490 which appeared in prototype form in 1922. The exceptions are Titch Allen's articles in Motorcycle Sport in the 1970s, and Bob Holiday's The Norton Story.
It seems that the Norton offerings in 1919 were most likely the Big Four 633 cc and the Model 16 490 cc as all-chain-drive machines (with mention of belt final drive option), together with the direct belt drive "sports" models. It seems that for 1919 and 1920 the Big Four and Model 16 shared the "high ground clearance" (6in) frame, perhaps most easily identified by its fuel tank which was somewhat upswept at the front. In 1921, the 16H was introduced with a lower frame (4in clearance) and the "speed" tank of the style fitted to the belt drivers. The 16H resembled closely the machines raced in the 1920 TT.
The 17C (C for colonial) introduced for 1921 kept the 6in clearance frame of the Big Four, but differed from it in having the 490 cc engine, lightweight Druid forks and chain guards on the top runs only. I assume then that the 1919-20 Model 16 and the 1921 Model 17C are quite similar.
Some dating points: