That's a very nice British Service Revolver. This model actually played an important role in the transition of S&W to military production at the end of the Great Depression.
The British advanced a million dollars (or thereabouts) to S&W to develop a 9mm Light Rifle. When the prototypes were delivered, the British rejected them -- long story, details available elsewhere. S&W would have bankrupted if forced to pay back the advance, but following some serious negotiations the British agreed to take lots of .38/200 service revolvers. S&W turned out tens of thousands of these in 1940 and early 1941, and then Congress passed the Lend/Lease program under which America provided lots of war materiel to many countries -- but primarily the Brits -- in exchange for the right to (among other things) set up military bases on British soil. Revolvers supplied under Lend/Lease were considered to be American property on loan to Commonwealth forces, but the previously produced British Service Revolvers like yours were contract goods purchased by the British and owned by them from the outset.
They are a special and very interesting class of S&W revolver. Congratulations on having found one in such good condition.
Don't let anyone tell you the .38 S&W load is a weak one, either. It may be measurably less powerful than a .38 Special, but you would NOT want to be hit by one. The .38/200 was a standard British military handgun cartridge from before WWI until the mid-1950s.