Carl Zeiss Jena Sonnar 135mm F/3.5 portrait lens. M42 mount, multicoaded (red markings.) The most desirable iteration, and I got it for about the going rate ($125). In spite of its ancient optical design and rather slow maximum aperture, the this is a muchly loved lens. It is prized for its razor sharpness even wide open, and sublime bokeh. That said, I'm am an IDIOT for impulse buying yet another manual focus 135mm prime. This makes ...four that I own, with one more that I want. There's a Russian made Tair 11A 135mm F/2.8 auction ending soon. It has a 22 blade aperture. That's more than even my behemoth Myer Optic Orestor 300mm F/4, which has a paltry 19 blades.
EDIT: I just bought the Tair: