Brucek is correct in that the ammunition was for the Canadian Navy for their .45 Colt New Service revolvers.
I agree that this is for Royal Canadian Navy use. Skeeter Skelton once pictured one of those 1940-purchased .45 New Service guns in, "Shooting Times."
The ammo is collectible and worth far more than it is for shooting.
Ian Fleming had a .45 New Service and was shown with it in a profile on hm in, "Life." He mentioned James Bond using a "long-barrelled .45 Colt" as his car gun when he had the .25 Beretta on his person. Bond supposedly had that .45 while doing Secret Service ops in WWII. I think it reflected what Fleming saw being used then, whiie a Naval Intelligence agent. Some of those guns may well have been from that 1940 order for Canada, or from similar British purchases.
Because .45 Colt was not a standard British military caliber, many of tbose guns were probably diverted for special uses.
Also, Colt autos in .455 (auto) were eventually diverted to RAF Coastal Command, to keep all the odd caliber ammo in one command. That would make sense for .45 Colt guns, too. Just a historical thought.
Anyway, don't shoot that ammo. It is part of history, and hard to find today.
May we see a side view of a cartridge? These seem to be FMJ bullets, as required by the Hague Accords for formal warfare.
NOTE: those 12-round boxes reflect the normal ammo issue for a revolver in Brit WW II service. Just one load and one reload! Special ops (SOE) men had more ammo, and I think paratroopers and Commandos did, including SAS, etc. They made more extensive use of handguns than did most troops.