I have three Geha's. On two of the '60's 790's (KM & OEF nibs)there is a green 'button' on the bottom, just like the school pen nibs I have. They do not fit the 790.
On my 725 Goldschwing there is a black 'button'.
One pushes the button in, to to release the 'reserve tank'.
When you twist the piston all the way to the front to start loading the pen, it automatically, pushes the button out.
Having taken a spare school nib apart a tad, I'd wondered real hard, how that tiny little slot in the green rod, could hold two pages of ink. Thanks for the drawings to show the additional area in the feed that held the real two pages of ink.