Blankenberge is next to Zeebrugge.
The Hafenschutzkommando (Harbour-protection group) Blankenberge consisted of three batteries.
Commander was Korvetten-Kapitän Lenne (substitue Kapitänleutnant Schwartzenberg)
There were three batteries:
Batterie Groden (Korvetten-Kapitän Lenne and Kapitänleutnant Schwartzenberg) with four 28cm howitzers and north of them a few 21cm grenade launchers
Batterie Mittel (Oberstleutnant z.S. Ernst and Oberleutnant d.R. Niebert-Krüger) with three (maybe four) 10,5cm canons
Batterie Hafen (Oberleutnant d.R. Baustert-Möhlenbeck) with four 8,8cm anti-aircraft guns on concrete-platforms.
I think, the canon is too short for being an anti-aircraft gun. I assume, the description of the card is wrong, and we have one of the 21cm grenade-launchers to defy the coast against landing-troops in the battery "Groden"