The Greener Light, MKII, Harpoon firearm was made by the well-known arms maker, Greener of England and distributed by the Navy Arms Co. here in the United States. This is the harpoon firearm that was fired off the bow of the boat in the 1975 “Jaws” movie. Yes
"We’re going to need a bigger boat"
Kynoch: The cartridges pictured below where produced by Kynoch of England for use in the European continent for the Light Green Harpoon firearm. Note the light green wax on the top of the cartridge to seal the cartridge water tight!
Navy Arms: The cartridges pictured below where produced by Navy Arms for use in the USA for Light Green Harpoon firearm. Note the the “Spl Tool” on the head stamp on the modified “WRA” Winchester Nickel plated “38 Special” case.
This is about all the information I have found on the firearm and cartridges, which is fairly sketchy, so please feel free to add more, if your more familiar with the subject.