Leitz Summar 2/35mm lens from Aristophot
This uncoated lens came from the Leitz Aristophot 4x5" (9x12cm more precisely) photomacroscopic stand, type I mostelikely since they are the brass version.
http://www.macrolenses.de/ao_detail.php?AdonsNr=200 showing also the fitting condensers etc.
(Thanks to Dr. Klaus Schmitt for the input).
Good used condition.