Most of the Nortons were provided with speedometers made by Smith, with a maximum reading of 80 MPH. (numbers given on the number plate are: S.433M or S.433B/EX or S.434B/EX).
These speedometers work on a chronometric principle were speed is measured in small increments instead of continous movement. The intermittend movement of the needle therefore does not indicate a problem, its just how the instrument works. Speedometers used by the military were, not provided with trip counters or internal lighting provisions. Those gadgets were reserved for the civvy market. Norton Assembly book entry specifically states  Speedo "without trip". 

Two types of speedometer connecting drives have been identified, one with a round protruding connection and one with an internal square connection. The round protruding connection requires a large diameter speedometer drive cable whereas the internal square speedo uses a rather modern looking speedometer cable of a much lesser diameter. The illustrated parts list picture shows the early type (large diameter) cable/connection. The length of the earlier cable is not available at the moment, the later type is 25".

The bottom part of the speedometer cable was identical for both speedo's consisting of a flat protrusion to be inserted in a slot and screwed secure onto the speedo gearbox drive on the frontwheel brake plate.

It is not clear yet as of when the early types were changed to the later types. Pictures of bikes from contract V7353 still show the thick cables, which date them at least upto end of 1940, and possibly even later than that.

Early motorcycles had a centrally mounted Speedometer. The earliest contract I can find with a lefthand mounted speedometer was C6127 (based on pictures) of which the first bikes were delivered in May 1940. The spareparts list for this contract however still refers to the centrally mounted part number 3884.
The spareparts list of V7353 shows the left hand sided mounting (spare part no. 4114) superseding the centrally mounted version.  This more or less indicates that at least all bikes with frame numbers above W26000 were provided with the left hand mounted speedometers. 

Despite the apparent water tight appearance, these instruments do collect water inside. Drying out can help prevent damage to the internals. As can be seen in the picture, my milage drum has suffered from corrosion, and I did once repair a malfunction by cleaning and drying out the inside.
The "Motor Cycle" magazine of April 1992  shows how to execute a revision/clean up of this type of speedometers.