[Lynn Eades]

Tom, you need to remove the lid (as per Michael R's picture and put it in a vice. Maybe you can hold it by the two cast horns. Movement of the gear change mech. toward or away from the housing (lid) should be relatively easy across the three selector fork slots while all are in the neutral position. If this is not the case, I would pull the gear lever from the housing to establish why. It has to be at the lever.
under the nut is a washer and spring. Remove them and the lever and check the small pin that engages the slot in the lever. It is fitted from the inside.

On the gear change clamp: it will be a post service mod. The early ones had an adjustable left and right threaded bolt like the ones in your linkage adjustment, only bigger. They were a dumb idea and by the time of the building of the U.C. I think they had moved on to the fixed arrangement as Rob and others have mentioned