Thanks so much for your replies everybody!
That looks very close to mine. The construction is slightly different: on mine, the heat can only exit the bottom though , not the top. It exits via the smaller round holes and the two wider slots in the hex-shaped bottom (the motor still runs BTW). This low-profile construction does make me think it was specifically built for under-seat placement.
I heard the following from a shop in Germany: ' this special heater was used in 170ies and 300 models as additional heater for cold countries used under the RH driverseat.'
Regards,
Klink