In use, the pump sat in the top of the tank between your knees*. The plunger was spring loaded upwards, and was pushed down by the rider every few miles and certainly at the bottom of a hill. There was a drip regulator and perhaps a glass bowl on the outlet right-angle, and the little catch was to hold the plunger down when you parked outside the bakers so that the assistant could get flour on your undergarments again.
In your dreams
*PS, fortunately the effect of the brakes was to produce a faint rubbing noise, nothing else. This was fortunate because of the position of the plunger in relation to aforementioned garments.
Tim