Agreed, these had to be regular bombs or shells. Minelayer would probably avoid the coastal batteries and fly a little west of the shore to stay out of reach as long as possible. In my opinion flying over Groningen with Den Helder west, the Frisian batteries north, many more south and Germany East, including the Kiel area would have been suicide for minelayers. The only plausible operation I can think of is a straffing, a milk run or a bombing mission. They had to be deliberately ordered to go to such a heavy defended area. I would try to see if Twente , Groningen, the Dutch Frisian Isles or German Ost Friesland was targetted.

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