The classic fast 90mm for Leica R — the pre-APO portrait telephoto.
The Leica Summicron-R 90mm f/2 is the R system's classic fast short telephoto, produced from 1969 through the 1990s in 1-cam, 2-cam, 3-cam and later R-only and ROM variants.
Optically it is a five-element double-Gauss derivative built for portrait distances: heavy brass-and-glass construction, a built-in sliding hood, and the long focus throw typical of Leitz manual telephotos. Wetzlar and Canadian (Midland) production both exist.
Its significance is as the affordable entry into fast Leica R glass: it renders in the classic pre-APO style and sits well under its successor - the APO-Summicron-R 90mm f/2 ASPH. of 1998 - in price, which is exactly why the two must not share a master record.
Used buying: check which cam configuration matches your body (1/2/3-cam vs R-only), look for haze in the rear group and oil on the aperture blades, and confirm the sliding hood still locks - Canadian examples price identically to Wetzlar in practice.