Leica Summicron 90mm f/2 I

The Leica Summicron 90mm f/2 is a fast short-telephoto M-mount lens made in Canada from 1959 to 1980. It uses a 6-element, 5-group double-Gauss optical design and was one of Leica’s key high-speed portrait and telephoto lenses of its era.

Across its production life, the lens appeared in several different types. Early versions, such as the 1960-style chrome lens, used mostly chromed brass construction, had 15 aperture blades, and an aperture ring that operates in the earlier “backwards” direction. Later versions from the 1970s changed to more modern cosmetics, used more aluminum in the barrel while retaining brass helicoids, and typically had 12 aperture blades with a normal aperture-ring direction. Despite these handling and construction differences, the M-mount optical design remained unchanged throughout production.

Good source, if you're interested in all the lens variations.