Here’s the patent for the usus.pi: WRITING IMPLEMENT FOR AN EXCHANGEABLE WRITING INSERT
Addendum: I’m not sure if this is really the patent for the usus.pi or – in other words – if the exact mechanism of that pen is covered by a patent. – Here’s another patent by Hilmar Nicolay: Writing instrument for writing or drawing has upper and lower parts of shell fixed in position relative to each other by magnetic force (I think this relates to the usus.i).