Yep, that's the gadget - I found mine this afternoon quite quickly in the Upper Cretaceous deposits. It's a tool from CH4D, for putting a crimp cannelure on lead bullets. However I think you need to make a plug to fill the case back to just in front of the cannelure and keep the neck round*, otherwise if you put enough pressure on to make a good groove, the neck might become distorted. I just dived in with an old, fired 45ACP case, not perfectly round, and that's what happened when the case slipped off the rolllers. But up to that point the case had been getting "groovy".
*And the plug can be made with a step to hold the case so that the groove is pressed in the right position.