This is an Exceptionally rare, 1957 Production 5cm, 2.0 Ridged Summicron in black Paint #1465745.
The batch of lenses in black paint from 1957 are typically found on the Leica MP Black Paint from the same year. There were only 138 MP's produced in black paint, so one can deduce that there were fewer matching 5cm Summicron lenses sold with these cameras, making this an exceptional find.
Jim Lager references this lens, from the same 1957 production batch, on page 167 and 168 of Volume II of his reference set. Some links to recent sales follow: