According to History of Smith & Wesson by Roy Jinks, production of the British Service Revolvers began in March 1940. In September of 1940 the firm concentrated efforts on only this model to meet the needs of England's War Effort. By Feb 27, 1941 production increases allowed for the Military and Police model (in 38 Special) to be placed back in production. From then until March 29, 1945, British Service Revolvers (38 S&W) were made along with the US Victory Models (38 Special).

So, there was a limited time when only British Service Revolvers were produced. I agree, a factory letter is the only way to verify exact production details for a specific revolvers.

Hope this helps.

Steve