It is a Renold chain-maker, as opposed to the common chain breaker. It only works on one size chain and I used it the other day to add a link to the chain on one of my bikes. This is for 520 chain, and I have one for 530 also, I don't know what other sizes they came in, Renold does not seem to have any info on them right now.
The side plate of the chain is held by the little plate with the chain attached and the chain you are working on is put across the mandrel, closing the screw presses the plate on. Then you open it up, remover the little press plate and two stakes are uncovered which stake the plate in place when the screw is run in the second time.
Ex Berliner/Kegler tools.