The 1988 Canon EF 600mm f/4L USM was the EF system's first super-telephoto flagship, launched in the autofocus arms race that followed the EOS 650 and built for press and wildlife work.
A 6kg magnesium-and-fluorite design with ring USM autofocus, it set the pattern every later 600/4 refined; there is no image stabilisation - that arrived with the 1999 IS version.
Its significance today is as the affordable route to a 600mm f/4: clean examples trade at a fraction of any IS generation, and the optics remain seriously sharp on modern bodies.
Used buying: check the fluorite front group for internal cleaning marks, confirm AF drives smoothly across the range (USM units of this age can hesitate), inspect the drop-in filter holder, and remember there is no IS - a listing claiming IS at this price is a different lens.