After posting this, the light bulb came on (dimly) that many people may have never seen one of these before. So, here's how they work:
Powder is poured into the hopper:
The powder then fills the lower reservoir by gravity:
The drop tube has an adjustable slide that locks into place with the thumb screw. The threaded nut and thumb screw keeps the slide in place. The slide determines how much powder is in the drop tube:
The drop tube is then
offered up to the powder measure. Normally, I'd say you just "...stick it up there". But "offered up" sounds pretty classy...kinda' British.
Actually, I rebuilt some SU carbs for an MG in the past and the manual said that you "..offered up" the float bowls to the main body of the carb. And I've been waiting all these years to use that term....
The handle is pushed back and the lower reservoir slides to the right...which uncovers the hole over the drop tube.
The drop tube is removed:
The power is dumped into the case: