Not a standard part methinks. Basic camplate design is the same on all boxes, so this applies to the later S/A box as well.
The face of the camplate should not press heavily onto the selector forks when the selector rod and inner cover are in place. Conversely if the camplate can move towards the inner face of the case, the depth of engagement for the pegs in the camplate tracks is less, so you need a happy medium. Enough freedom to rotate, close but not binding, but not too loose to be sloppy. Standard build with new parts all was fine, no adjustment needed or provision for. Looks like a P.O. mod of some sort. I have had the odd camplate that was too thick (factory second?) that caused a problem, solved by a bit of detective work. viz....
The down changes in the box are assisted by the plunger and spring, too tight between forks and camplate and you can change up OK, but it won't change down, binding too much for the plunger to nudge the cam.
With this particular gearbox, look for wear to the camplate, gearbox casing and a bent selector rod.
As a hint, fit the grubscrew last, to avoid breaking a chunk off the casing when you accidently smack the end of the seleector shaft when fitting the inner cover. Selector mechanism will work without the gears for testing purposes. Early version of the plunger box uses flat clockspring on the kicker, no grease nipple on clutch arm. It may also not have a sprocket oilseal. Good news is that later sprockets and oilseal are a straight swap as a pair.
Swarfy.
