This one is a 20 Gauge shell extractor. The 20th Century paper shells tended to swell in the amazonian humidity. And chambers tended to get crusty/rusty with the hands off approach to maintenance abided by the folks convinced it was the veneno ("poison") in the barrel of a shotgun that imparted lethality to the shot. I never owned a 20 gauge (not even to this day) and don't recall who gifted me the blade. But have always thought the idea intriguing. Being able to slide the extractor cutout over the shell rim and give it a solid pull beats trying to find something the right size and stiff enough to get the job done out in the jungle.

Gunner