Reply To: mystery vang
At a guess I would say it is an upside down cascade system similar to what the Fireballs (and some other classes use). The rope runs up from the mast, through a block on or in the boom, there will be a purchase system somewhere (possibly in the boom on this boat).
I used a cascade system on 2100 when John Ball owned it and thought it was very effective. More than others have suggested.
