Remember when I said
Köln had been completely 'pimped', because it's mostly downhill from here. By 1938,
Köln had made some rough expirience in the stormy bay of Biscay during her Spain missions and her inner structure had to be reinforced. To compensate for the added weight, the aircraft catapult had to go and the massive crane had to go. However, another grid crane was installed a short time later, to better handle the larger boats.
After the Invasion of Norway, a degaussing cable was installed, but the larger crane was once again removed and replaced with the old, smaller version. Also, the lower searchlights and aft torpedo tubes were removed and the eagles on both sides at the stern were reduced to one on the back, like any other ship. More significantly, however, the
Köln served as a helicopter testbed: from 1940-1941 she carried a wooden platform on turret >Bruno< that allowed the landing and taking off of either a Fl-265 or Fl-282.
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Any help and source material is always welcome.