Ok. .  It's a 'hoop' for a Merlin Rocket sailing dinghy.  Two uprights approx 900mm, cross bar at top approx 400mm supporting a short section of mainsheet track and traveller.  Must be able to withstand shock loadings (and me hanging on to it when I lose control of the boat!) of approx 70kg. Hope that makes sense.  Here is a picture of one without track: 
https://goo.gl/images/i59tCn