Kurt Sperry asks: "....What happens if one tries to shift across a 4 speed block or ...3 speed with a 5 speed derailleur of this sort? And why not simply limit the motion's travel (obviously no mean feat to accomplish) to suit whatever is required on the day rather than making multiple incompatible pieces?..."
after Neil Foddering said "...Because the range of travel across the sprockets on the block on this type of derailleur is dictated by the length of the spindle along which the derailleur jockey cage slides, and therefore, a different length of spindle is required for 3, 4 and 5 speeds...."
The bicycling industry does many strange and wondrous things, and maybe that's just the way it is for this. One thing comes to mind ... if you use, say, a five speed Cyclo Benelux Mark 7 rear derailleur on a three speed freewheel, you have to limit the horizontal travel of its pulley cage to keep it out of the spokes. On a three speed freewheel instead of a five speed freewheel, you will have to limit it much more. You do this by unscrewing its five-speed-length threaded adjusting sleeve (which surrounds the operating shaft controlling horizontal movement) much farther outboard than usual for a five speed freewheel (and then lock it in place with the adjusting nut). The result is that your rear derailleur's adjusting sleeve will stick out really far, and hit on even more things than your Mark 7 rear derailleur does already. Your Cyclo Benelux Mark 7 rear derailleur will look like a foot peg on a freestyle bike.
Peter Brueggeman
La Jolla California USA
4peebee(at)peterbrueggeman.com