An engine number in 1963 looked like this B41/1/501572 WSO , which means that the engine was the 1572 made the WSO means for export.
Click the picture to enlarge it.
In this case, this is that engine and we know it will date onwards and from May 3rd 1963 when the Imp was released by Rootes Scotland.
I asked Colin, an Imp specialist in the UK if he knew if the engine number had the WSO (export) or HSO ( home saloon) stamped when it was made, or were the letters added when the destination of the car was known/
To my surprise I find that Colin worked in one of the Rootes factories down south of Scotland!