I was intrigued by the idea that a British citizen might use ammunition manufactured by an American company. I would have assumed Gallagher would have used British ammunition, such as
Eley. However, I found the following statements
at the Remington History website that provide one way this could conceivably have happened:
"1934 - On May 15, 1934 Remington purchased the
Peters Cartridge Company. The Peters Cartridge Company operated an ammunition facility located in Kings Mill, Ohio."
"1946 - In November, 1946, Remington closed the Remington Arms Union Metallic Cartridge Company, Ltd., a wholly-owned English subsidiary. This subsidiary operated a shotshell loading plant at Brimsdown, England. Operations consisted of loading empty paper shotshells, imported from the parent company, with powder and other components.
This company also handled importation of rimfire and centerfire ammunition and sporting firearms from the U.S."
So the presence of Peters cartridge cases doesn't rule out their having been purchased and used by a British citizen.