Kevin,
On older Super7Bs ,there is one of these on the outside end of the gearbox layshaft, next to the leadscrew, as you have noted; the only other one in the headstock is on the backgear cluster, and holds the backgear twin gear onto its shaft on the backgear eccentric. Partially hidden under the bullwheel.
However, in some manuals this is shown as a circlip instead of the screw and washers.
Apart from this, am as baffled as you are.
Phil