I just did a search on this but I couldn't find anything on the perimeter defenses, I'm really curious as to what that is, with that concrete pintle there?

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This is a pair of Dutch resistance crystal radios, built into a small metal can, and a matchbox. The image is from a postcard bought at a Dutch Resistance Museum in Amsterdam. The matchbox is marked in Swedish, but Swedish matches were sold in Holland for many years..
Scott