Neat. Were any of them Danish by any chance? I keep running into references to this early design concep for the RDN from before they bought Dykkeren
The design of this submarine was heavy based on the Dutch submarine ''Luctor et Emergo'' which later become the O1. It was a submarine based on a Holland (II, I believe) frame, but with Dutch/German engineering and as well in cooperation with an Austrian engineer. As the Belgians didn't show interesteds anymore and ask for minelayers instead, this model would have been (acourding the building contracts and design files in the archives) aswell offered to Denmark and Sweden. Sweden would never get back to it, but there was a letter inside that a Danish officer came down from Copehagen along a German officer to visit the Dutch vessel and be a part of the public to look the ship doing submerge trails in the harbor. The Danish also followed the next type, which was offered to Denmark and Sweden as well. But these details I will have to research more, though. Archives are a mess currently.