Used but in good condition the classic 1972 pattern GS Bergen rucksack.
The definitive bergen of the 70s and 80s, made of a Butyl nylon coated material and mounted on a steel frame with quick release shoulder straps.
Not the most comfortable bit of kit but, along with the SAS PARA type the only surviving bit of kit from the ill fated 1972 pattern equipment which was designed to be more easily decontaminated than webbing.