16 h primary and secondary chains 1930's

Primary - 1/2" pitch x .305" No of links 74

Rear - 5/8" pitch x .25" No of links 91

Magdyno - 3/8" pitch x .155" No of links 42

Chain adjustment Front 3/8" whip

Rear 1/2" whip

Magdyno 1/4" whip

This is according to The Norton 16H Maintenance Manual and Instruction Book. The manual is undated but I would think it would apply. Others on the NOC site are better qualified to recommend the best sources. When I needed a primary chain for my Model 18 of similar age I went to Russells in Wandsworth who supplied a Renolds chain.. some say they (Renolds) aren't what they used to be though.. 

The civilian bikes had slightly different gearing from WD, with one less teeth on engine and one less on rear brake drum. So you might need a chain splitter and add or subtract a link from the WD numbers. In old money  proper Norton front chains were always 1/2 × 5/16", and rear were 5/8 × 1/4".

The manual I was referring to was downloaded from www.wdnorton.nl website which, if you haven't found it already, might be generally helpful, but as David points out might be misleading where there are differences between the military and civilian versions. 

There is so little clearance between both primary and final drive chains and the chaincase inner that it is important to use chain made to the original outer dimensions and not simply suitably sized rollers and inner dimensions.

Iwis and Regina have a good name...Availability, including half links can vary.

https://www.ebay.co.uk/itm/Iwis-1-2-x-5-16-Chain-110-pins-with-connecting-link/373371150384?hash=item56eea7a030:g:PUoAAOSwNCtdHmRa

https://www.ebay.co.uk/itm/Regina-520-Chain-Non-O-ring-Gold-X-120-Inc-Connecting-link-5-8-x-1-4/373164677746?hash=item56e2591a72:g:tyQAAOSwc~dduWnV