I originally had a Series 1 set that I received as a gift in the 1970s, along with a Series 3 set I received a few years later. I since bought a Series 2, the Elektronik AG set (1103), the Radiotechnik set (1110), and the Ausbau-System Radio-Röhrentechnik set (1120). I would very much like to aquire more sets, but as far as I can tell, delivery is only available to Germany these days.
I did build the semiconductor based (NPN and PNP) flip-flop circuits from the manuals, which is where I got the idea of re-implementing them with vacuum tubes.
I’m also considering building a ferrite core memory like the ones in Günther’s amazingly complete listing, which are presumably not obtainable.
I did reimplement the bistable one with fixed resistors instead of potentiometers, it works well. I have the “toggle” version just barely working, but it’s not reliable yet: higher value resistors or capacitors would help, but all of the higher value capacitors I have are low voltage units (I may try a couple of steck blocks to substitute others), and the only resistor I have that’s more than 100kΩ is a single 2.2MΩ unit.