i use a similar peice of kit in work for clamping tube or round bar down instead of a vice. the handle sitsin a threaded hole, with chain facing up, the chain goes over the work, and slots into a groove on the base of the vice, as tight as you can get it (closest link to slot in groove) then you wind the handle out to tighten, and the locking ring to stop it slackening off. theyre not identicle but they do look similar
but yours has a few other bits on it, so it is likely to be for something else, maybe for marine use