I have a drawing somewhere of the internal construction of these 1938 silencers...They were apparently very quiet and this was becoming more and more of a concern at the top end of the motorcycle market in the late 1930s. The silencers were so large that the pillion footrest lugs had to be extended. They were available in left and right hand versions for the twin-ports...and in view of the fact that all Nortons were available to 'Comp' spec with raised exhausts, it may well have been that some were delivered with two of these fitted at high level !
The bike in the photo looks to have been an impressed 1938 bike as it has 'combination' handlebar levers, an illuminated speedo and the civilian type headlamp clip as well as the "Cow's Udder"