Jules,
As others have said, REME was not formed until 1942 when it incorporated elements of RAOC and RASC. I think you will find that MT vehicles were normally maintained by Royal Army Service Corps workshops or LAD's. The sign for RASC is red and green square, divided bottom left to top right, with red to top. I have no info on what number to use for 1941, the numbers changed during the war period and also differed between infantry and armoured divisions.