Sigma's 135mm f/2.8 was one of the most common third-party telephoto primes of the manual-focus SLR era, sold as an affordable, fast alternative to camera-brand 135mm lenses for portrait and sports use.
The lens is a fixed 135mm focal length with a maximum aperture of f/2.8, manually focused via a long-throw helicoid. Sigma manufactured it for Canon FD, Nikon F, Pentax K, Minolta MD, Olympus OM, M42, Konica AR and Contax/Yashica mount SLRs, using multi-coated optics in an all-metal barrel typical of primes from this period.
The f/2.8 aperture gives a shallower depth of field and brighter viewfinder image than variable-aperture zooms covering the same focal length, making it a popular portrait lens; wide-open sharpness is workable but improves noticeably by f/4-f/5.6.
A cheap, easy way into fast telephoto primes secondhand. Check the focus helicoid for smoothness and grease consistency, inspect for haze or fungus in the long barrel, and confirm the aperture blades stay clean and free of oil.