It is an Isco-Göttingen Iscovitar 1:2.8/50, also known as Isco-Göttingen Edixa-Iscotar.
Yes, it is a triplet (i.e. it contains 3 elements in 3 groups). Something like a Cooke triplet:
This is a cheap (for its time) lens with plastic body parts, it feels cheaply built. It reminds me of
the Revue 24/4 lens.
The aperture has only 5 blades.
As you can see, my copy obviously needed service or even repair. But so far I only use it at f/2.8. For more stopped down apertures I have other lenses.
There is an "automatic" aperture pin.
But you can switch from the A to M mode to control the aperture directly.
The aperture ring has click stops at f/2.8, f/4, f/5.6, f/8, f/11, f/16.
There is a screw thread near the aperture mechanism.
As far as I know, this is for a threaded cable release.
(source unknown)
Maybe for macro photography with bellows (with double cable release)?
The filter thread is 49mm. The front part of the lens (with the filter thread) rotates during focusing. And this simple focusing mechanism is the most interesting part of the lens.
When I started using the lens, its image production at close focus reminded me of
the Riconar/Rikenon 55/2.2 lens. Very similar soft focus effect. And the reason is the same! The simple focusing mechanism moves the front optical element only.
And again, the minimum focusing distance is about 0.8m (too far for a 50mm SLR lens). At the shortest distances, a soft focus effect would be too much. And of course, manual focusing is quite difficult.
