The RL on the case would indicate Royal Laboratory, Woolwich, England.
The stamp "51" is probably a 1951 manufacture year.
The 2Z means it's a Mk-II load with nitrocellulose powder.
Boxes may be marked "Cartridge, Pistol, .380" Mk II" or ".380 Mk IIz", firing a 180-grain full metal jacket bullet.
Kynoch produced a cartridge with the same dimensions as the .38 S&W but with 2.8 grains of "Neonite" nitrocellulose powder and a 200 grain bullet. This combination gave a velocity of 630 ft/s at the muzzle, and over 570 feet per second at 50 yards.
Revolvers chambered for .38/200 may also fire .38 S&W (AKA .38/145), .38 Police Positive. Always exercise caution when using ammunition designed for more modern guns to different specifications.
If you can disassemble one of the rounds, it's probably loaded with an early smokeless nitrocellulose. It's always possible you'll find multiple "strings" of some kind of
Cordite powder instead.
Looks like Jonnyc beat me to it.