I had a quick look on the German Patent Office patent database (DEPATISNET) for MK sockets (The UK patent office is down for maintenance this weekend, and DEPATISNET has an english-language interface option).
Results summarised below.
As well as the two types mentioned above,there was a separate patent covering specific use in a 2 pin socket.
re #156, there are also three MK patents relating to arrangements requiring the simultaneous insertion of all three pins. I don't know if any of these latter designs ever made it into production. I have certainly never seen any. Perhaps they did make a trail run of some for evaluation.
The links take you to page 1 of PDF files. To view an entire pdf, click on the button at top right marked "Volldokument laden".
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GB 1438938 13A socket with shutter operated by 2 pins
https://depatisnet.dpma.de/DepatisNe...B000001438938A
GB 1460001 2 pin socket shutter operated by 2 pins
https://depatisnet.dpma.de/DepatisNe...B000001460001A
GB 1547989 Europlug-defeating 13A socket
https://depatisnet.dpma.de/DepatisNe...B000001547989A
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13A sockets with shutters requiring all three pins to open:
GB2068651
https://depatisnet.dpma.de/DepatisNe...B000002068651A
GB2260863
https://depatisnet.dpma.de/DepatisNe...B000002260863A
GB2261120
https://depatisnet.dpma.de/DepatisNe...B000002261120A