From Wikipedia:
https://en.wikipedia.org/wiki/.38-200
After a period of service, it was realized that the 200 gr (13 g) soft lead bullet could arguably contravene the Hague Convention, which outlawed the use of bullets designed so as to "expand or flatten easily in the human body". A new cartridge was therefore adopted into Commonwealth Service as "Cartridge, Pistol, .380" Mk II" or ".380 Mk IIz", firing a 180 gr (11.7 g) full metal jacket bullet. The 380/200 Mk I loading was retained in service for marksmanship and training purposes. However, after the outbreak of war, supply exigencies and the need to order readily-available and compatible ammunition, such as the .38 S&W Super Police, from U.S. sources forced British authorities to issue both the 380/200 Mk I and MkII/IIz cartridge interchangeably to forces deploying for combat.
The .380/200 cartridge was phased out of British service in 1963, when the 9 mm calibre Browning Hi-Power was finally issued to most British and Commonwealth forces. The Cartridge, Pistol, .380" Mk IIz is still produced by the Ordnance Factory Board in India, for use in revolvers of the Indian Army and some African countries.
Revolvers chambered for .38/200 may also fire .38 S&W (AKA .38/145), .38 Police Positive, and .38 Banker's Special cartridges, along with the .380" Mk IIz round, though caution should be exercised as always when using ammunition designed for more modern guns to different specifications.
From the British Imperial War Museum:
http://www.iwm.org.uk/collections/item/object/30027524
Description: introduced in 1932 as the military equivalent of the 200 grain .380 Webley Special cartridge introduced in 1929. This was a blunter, heavier bulleted load for the ubiquitous .38 Smith & Wesson cartridge, which was normally loaded with a 146 grain bullet. There was an equivalent 200 grain load in the United States called the .38 Smith & Wesson Super Police, and it is not clear whether this was originally a British or American development. The 200 grain bullet certainly increased the effectiveness of a hitherto rather modest performer used mainly in police and civilian self defence revolvers, and the British army considered it was an acceptable replacement for the .455 revolver cartridge. While the .455 was a formidable load at short range, the revolvers chambering it were large and quite heavy, and in 1922 the War Office decided to replace it. The .380 allowed the use of a smaller, lighter revolver but without an unacceptable level of recoil for the average military user. While round nosed lead bullets had been used by many combatants during the First World War, including the Germans, without concern, thinking changed during the inter-war period with regard to the compliance of such bullets with the St Petersburg Declaration and Hague Convention. Having provisionally adopted the adequate .380 Mark 1 round in 1922 and the Enfield revolver, designed by Captain Boys, to go with it in 1932, Britain decided to adopt a Mark 2 cartridge with a jacketed 178 grain bullet in January 1938. This was less effective, but the Mark 2 cartridge and Enfield, Webley, Smith & Wesson and Colt revolvers chambered for it were given widespread use by British and Commonwealth forces, various Allied forces equipped by the British and Resistance groups during the Second World War. It continued in British service use until 1968, and remains in use in India, and probably elsewhere.
Mark, stamped, case base (headstamp): K56 // .380 2Z
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