Regarding you later batch of questions (which are worth asking)
Fixed pitch vs constant speed: fixed pitch is pretty simple, opening the throttle makes the prop go faster, nothing too tricky there. On a constant spped unit, a governor adjusts the prop blades so the prop always maintains the same speed. at low throttle settings the blades go fine to unload the engine and maintain RPM, as throttle settings increase, the governor matches it by coarsening the pitch, loading up the engine and maintaining the same RPM. The Throttle butterfly valve however opens up and more charge is going into the cylinders as this happens, which registers as manifold Pressure (which is confusing in itself as on normally aspirated engines it is actually a vacuum, which decreases as throttle is opened!) hence when you open the throttle, RPM should stay the same but Manifold pressure decreases back towards 30 in Hg.
with regards to low wing vs High wing fuel systems, there is no reason at all to say low wings cant have a "both" position, not any reason why highwings have to have one. it all depends on how the designer decided to plumb everything up. for example, my aircraft has an on - off selection and no individual selection for each wing tank. Both wing tanks gravity feed to a header tank supplying the fuel injection. Cherokees and the like dont have a both selection, but that is more to do with the location of fuel pumps etc in the system - it just happens to be the way Jim Thorp wanted to do it, there are plenty of alternative methods.