This specific copy is equipped with a Canon FD mount. This is a manual-focus, breech-lock, or "New FD" (bayonet-style) mount system used by Canon SLR cameras from 1971 until the introduction of the EOS autofocus system in 1987.
"Underground" was a budget-tier house brand, likely a private label for a photography retail chain or distributor. These lenses were manufactured by third-party makers in Japan or Hong Kong and marketed as affordable alternatives to first-party Canon optics. The lens features Multi-Coating (MC) to reduce flare and is an "Auto Zoom," meaning it maintains aperture linkage with the camera body. While common, it represents the era's boom in third-party consumer optics that made telephoto photography accessible to amateur hobbyists.