It's a plumbing tool.
Use = eg: 1/2" tubing to connect to a toilet cistern, these days you will use a kingco nut and compression washer.
Croxon tool, you would slip the nut on the tube first, then the croxon tool would slip into the tube, 3 ball bearings rotate inside the tube, slowly expanding and putting a bulge in the tube to stop the nut from coming off. After removing the tool, wrap some hemp around the bulge, to take the place of the nylon, then tighen it up to seal the tube up against the 1/2" nipple.
Not sure of any other name for this tool.