I don't think there was a contract of Model 18's made for the army, the one pictured was more likely an impressed civilian bike. It looks to be a 1938 Model as that was the only year Norton used that silencer.
Have put ours for sale on this site 16h with m18 engine double stamp on the lug and also under the frame but no bore or stoke and engine stamp is a different type to the frame stamp any ideas?