So it took a while to receive the other 18 inch frame. Looks very encouraging, already adjusted the derailleur hanger and the set up with an ahead conversion looks also very good. Now on to preparing and painting the frame and fork. The frame seems to be in a better condition than the red one and no damage visible.
Derailleur adjusted or might just use a new one that I found recently.
The headtube of the 18 inch frame is much shorter than the 20 inch frame. I would say, shorter by 3 cm ora little bit more than 1 inch. The fork which came for a 20 inch frame thus has a lot of space for inserting an 1 to 1 1/8 inch adapter and clamp on a stem plus add a couple of spacers. I have an extra long expander to support the shaft, but i think this is not really necessary.
I am happy, because the 18 inch version is shorter and lower. Should fit me better.