Also known as the Meyer Oreston 50mm. Mine is the 2nd generation, single coated.
f/1.8-f/16, 6 aperture blades, click stops half f-stops
6 element, 4 group double-gauss
M42 screw mount, mine has the stop down pin JB-welded down. 49mm threads
Normal focus from 1.1ft to inifinity
Working distance on bellows from 4" to ~2.5" Magnification to about 3:1 (reversed)