The impending German invasion after the Fall of France in 1940 as well as wartime scarcity caused Britain to simplify the Pattern 37 Battledress uniform in order to save manufacturing time and material. The Pattern 40 version omits a lot of “luxurious” details from the Pattern 37 such as covering flaps for the buttons, exposing them, and pocket pleats. This version would be produced until the end of the war in 1945, and was replaced by the Pattern 49 after the war.
More information about WW2 era British Battledress variations: http://www.warrelics.eu/forum/uniforms/clarification-british-uniforms-uk-no-canada-australia-india-etc-just-uk-142932/#post473473
Tag faintly says made in “Jun 45”. Unissued.
Dark brown buttons (compared to lighter tan one on later dates).
Tag says made in “Sep 1943”.