Is it not this one?
http://www.mir.com.my/rb/photography/companies/Leica/Leica-M3/Leica-MP/MP_Anthracite_kit/index2.htm
Yours looks darker, but I have experienced that this finish can look very different depending on lighting. I saw the anthracite mp in person in Japan, and it looked darker and more purple than it appeared in photos. Either way, it's a beautiful lens you have!
Edit: Clearly it is not that one. It will be interesting to hear what it was actually for! I suppose it could always be a repaint job. Is there a special serial number?
Stuart,
I personally own one of the silver/gray anthracite 35 Crons that you linked to (but gave up the matching MP). It's a beautiful lens with a matching metal focus tab (instead of black plastic). Only 400 were made for the Japanese market. I had it on a steel gray M9 when Stefan Daniel paid us a visit a few months ago and he commented on it.
I've never seen a black version of this lens.... until now. Given that there were only 400 of the gray lenses produced, it could be reasonable to accept that there were only 60 of the black made, again, only for Japan. It has the feet markings in red, just like a black paint model, but it has the same matte finish as the gray anthracite lens. And, everything else looks the same too. It's just black.
Either way, it's gorgeous!
David