This shows the overall shape and size. The second hole in the arm is for lining up the other tool on the seamer. The tool is made to fit in a stake plate, not a hardy hole.
The other end of the tool matches the smaller die. Reverse it and put it in the close hole and it's all ready to go.
I think it says "J. Wilcox" "Portchester". Precurser to Pexto. If you'd like to contribute some background, send me an email, and I'll post it here. (With credit.)
A nice inch and a quarter scrolled thumbscrew holds and tightens the fence.