16H outer primary drive cover

You probably do not have a matching pair of cases.  There are 6 or more combinations due a variety of inner and outers made for the various models of both singles and later twins. In typical Norton fashion the chunks all look like they will match up but footrest, gearbox and frame fitting differences can make getting a pair for a certain model challenging.  If you look in the Singles spare books and/or the grubby attachments you will see what I mean. 

I bet you won't be the first Norton owner to do such a thing. But if your outer cover is not 16H, it must be from a less numerous type so might be worth selling or swapping if only you could find out what it's from...but life's probably too short to be ideal...

I had a similar problem with a 37 ES2 I built up.  The inner cover fitted correctly over the footrest mount tube but the outer cover hole was out of line by just over a centimetre.  To remedy this I filed the footrest hole in the outer cover to allow it to fit over the footrest tube.  I then made up a crescent moon shaped piece of sheet metal and welded it into the opposite side of the hole which now had a gap to the footrest tube, thus restoring the hole to round.  I was worried that it would look odd with the footrest mount now not concentric with the raised circular portion of the outer case where the tube comes through but by the time the washer, the nut and the footrest hanger are on, and partially hidden behind the brake pedal it is not really noticeable.  This worked for me but maybe your cases are out of line by more than a centimetre so it may not work for you.  As a previous contributor to the thread says, there are quite a few variations in the cases, maybe I was just lucky that mine were not far out.

Looks like a few members would benefit from a central clearing house where we could swap items we have for the particular model we do not have with those who have the opposite problem...