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M like Leica M6
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What patents would have been infringed by a third party lens? Methinks it was mostly the other reasons you listed. There were several makers of tele converters for Pen, so the lens mount definitely wasn't keeping lens makers away.
As far as I remember lens mounts are patented - the patent for the Leica M bayonet has expired in 1999. That also explains why many manufacturers have varieties of their lens mounts over the years... for example the Nikon lens mount: Pre-AI, AI, AI-S, then came the AF modifications and G lenses etc. Olympus is not bad at that either: we had the Pen mount, OM, E-Series, 4/3, mFT etc... this is not just a matter of progress, it's a way to make money 😀
Talking about money, the original adapters that allowed using OM, M42, Nikon or other lenses on Pen cameras, are horribly expensive. I consider having some adapters manufactured, that will neither be too complicated nor too expensive. If you had the choice, which lenses would you adapt to an FT?