Shilly, these telescopic sights were manufactured 1941-46. The 4(T) sniper rifles were used from 1941 through to the 1970s. The number indicates that its a Mk3. The metal bracket will probably have a rifle number on it, which will be one letter followed by a 5-digit number starting with a 3, e.g. "V37345" or similar.

By the way, there is a large arms fair taking place at Bisley in Surrey during weekend after next 22-23 October. There will be several Lee Enfield specialists on the trade stands there, including myself. If you happen to be around, bring the scope along and we can tell you exactly what you've got and how much its worth.