>I agree that the thing can't get much bigger with the ring preventing expansion.
The more I look at this, the more I think we're going at it the wrong way. I suspect the tool isn't for expansion, but for compression. Something light, like sheet tin, could be placed *under* the ring. Then driving in the wedge would make it conform to the depression the ring sits in. In other words, the wedge serves to press material into conformation of the ring and groove. I admit I don't see the utility of this process, but it does seem to fit.
We may never know what this tool really did. It has all the earmarks of a shop made, field expedient, limited use specialty tool, perhaps for a small run order. It's split to accept the wedge, not sawn. The split is not contained by a pin or a band. It doesn't seem to have been made for a life time tool.
The only thing I can think of for its use is some kind of tin work, something used after a piece of tin was worked into a tube.