Hi Robin,
Based on my Boxford the large double pulley has a grub screw and a keyway. The 4-step pulley has only a grub screw.
Both grub-screws tend to loosen and when this happens on the 4-step pulley you could well see the movement you describe. I haven't stripped mine down in years but IIRC this grub-screw bears on a flat on the shaft, hence a small amount of loosening gives quite a lot of angular movement.
Incidentally, there are 2 types of rear drive countershaft, basically as diagram G above. They're similar to one another but the later one is more compact but gives an overall higher speed range.
Edited By Clive Brown 1 on 30/09/2021 09:01:25
Edited By Clive Brown 1 on 30/09/2021 09:05:54