Yukio Horie→堀江幸夫, the founder of Pentel, he was also the developer of some early ball pens (those with fiber tip), but not these pencils. Around that time, he is often listed as the inventor, although it was actually someone else. This often happened in their oversea patents that time.
I don’t know who is the actual inventor of DE1926208 because I can’t find the corresponding Japanese patent of it. JP19680036098, this is not the standard Japanese format for patent number. Of course I could limit the time span and searched for it by personor company’s name, but the problem is that many patents from that period cannot be searched properly cause the text information was probably generated through OCR and there were many errors.
DE1926208 mentioned the structure of double knock and Mechanica. JPY 1975000827, is only related to Mechanica, the inventor is 関谷文平. Could he be the inventor of double knock mp? I didn’t see any direct description of it.