Gear Change Control Rod Adjustment
Ideally the rod should be set when the gearbox is out of the frame.

Set the gearbox in top gear (4th) and remove the inspection plate on top of the gearbox so that you can see the layshaft.

The selector dog on the layshaft will be sitting between 1st and 2nd gear but will not be engaged with either.

You will see a gap on either side of the selector dog, between the selector dog and the face of 1st gears dog teeth and between the selector dog and the face of 2nd gears dog teeth..
These two gaps should be set as near to equal as possible by adjusting the threaded rod, winding it in the direction required to do so.

The assumption is of course, that the box has been correctly assembled and the shafts correctly shimmed before carrying out this procedure.

If the box is in the bike and has not been recently restored but is problematic when engaging a particular gear, the rod can be adjusted slightly to alter the degree of movement within the box and an improvement may be secured (or not).

However, this is 'shooting in the dark' as you cannot actually see into the gearbox to determine exactly what you are doing and the problem may lie elsewhere anyway...Only move the rod in small increments, say one turn each time and keep track of how much you have altered it in case you need to put it back or make more adjustments.