Marketed in 1953, this was the widest lens Zeiss ever built for the Contax, and was the first lens for 35mm that had a 90 degree field of view. Some samples are marked as 21/4. It has a rotating collar around the rear element to protect it, which must be aligned with the red dot when you mount the lens.
The 21/4.5 mounted on a Contax IIa, with the standard 21mm finder No. 435.
It could also be used with the turret finder No. 440
The lens, as provided in its bubble
A look at the rear of the lens, including the rotating protective ring