I suggest you contact the (real) manufacturer (or design owner) and ask if you don't feel like figuring it out for yourself (if you are 100% comfortable poking around with mains voltage just use a series resistor and you won't harm the lamp- if you are not comfortable get someone else local to help so you don't harm yourself).
Clearly the mains voltage will be applied on opposite sides of that barrier! So that narrows it down to the commoned terminals and one of the other terminals.
This is an LED lamp, probably with a high value series resistor and maybe a half-wave series diode.
simulate this circuit – Schematic created using CircuitLab
It could be a capacitive dropper too but the German documents show AC/DC compatibility which points to a resistor.
The manufacturer appears to be:
Signal Construct GmbH
Tel. (49) 7233 / 9531-0
Fax (49) 7233 / 9531-29
N 048° 54´ 03,4"
E 008° 48´ 31,1"
I think the actual part number is 22H0289. A pox on distributors who hide manufacturer's information.