This is the legendary Leica (or Leitz) 21mm f3.4 Super Angulon. For years it was a must-have lens for all Leica photographers.
The key to its success was simplicity. It's a true wide angle. Look at its rear element - when mounted it almost touches the film (sensor). In the 1960s the introduction of the single lens reflex camera forced lens designers to rethink wide angles. They were forced to shift the rear element forward to accommodate the reflex mirror. This inevitably compromised performance.
This lens will still perform almost as well as anything built today. Quite an achievement for an optic designed 70 years ago.