Kudos for Frustration-Free Packaging, me just fine. Now for the product. The extension has been through its first acid test. 2x approximately 6 hours of continuous use of an electric cool box of "Mobicool". The socket side has indeed become quite hot but there were no problems. Just in case, a replacement fuse for the extension was provided.
Two added little niggles I still found. First, if the fuse blows it out you need very strong hands to plug untwist. Better to use two pairs of pliers. Second, there is no feedback when said backup fails or you can not determine without opening the plug securing or their function. Why only 4 stars.