Here's a better picture, a friend of mine who often works with vintage (analog) electronics told me it's a "flyback/recovery diode" (Freilaufdiode in German) which would make sense as it's connected to a relais.
Picture was taken with flash activated on my smartphone (which is the best I can offer). Size is slightly smaller than a common leaded resistor for signals (0207?).
Also attached is the (partial, temporary) schematic I got so far, U1 and U2 are the parts in question, the green line underneath corresponds with what I interpret as the "white marker" for the cathode.