This is the towing arrangement for a 1/2 ton trailer, made in similar numbers by both Sankey and Brockhouse, and as far as I know only for the UK armed forces. As Richard quite correctly mentions, the idea is it can swivel around 180 degrees, allowing towing by either larger or smaller vehicles. It has twin brake actuator arms, one each side of the draw bar, which (assuming the trailer hasnt been messed around with) makes it a Mark 2, and probably dating from around 1955ish to 1960ish. If it only had one actuator arm, which went through a slot in the middle of the drawbar, it would be a Mark 1, from around 1952 to 1955.
It probably also has narrow dish split wheel rims, as fitted to 88'' MOD Land Rover Series 2As of the time.
Has it still got its ID plate, with its MOD reg no?
Hope this helps.