In a water cooled system there usually is no 'tank' as we know it. It has to be factored in that these were burgeoning systems ,perhaps no-one knew that water boils 3 x faster and 3 x hotter under pressure when these cooling systems were first designed. You can see some weird stuff on WW1 aircraft.
Tank may have been a suitable word in that time period to describe the radiator(regarding the manual you quoted ) . You are right in the assumption that the filler cap would be the highest point , factoring in the properties of water. Start there and figure the system out , there aren't a lot of components. Rad/tank , lines/hose' , water pump , cylinder jackets , temp guage and that's about it.