Although numbers 1 to 294000 were issued to the RASC, that does not mean that the person holding that number was in the RASC. My uncle (2032517) was called up for WW2 to join the RE, after a short time later he joined the Essex Regt, he then transferred to the Corps of Military Police (later the RMP) and became their RSM. He retained his RE number throughout his long career.
I doubt that GW was RASC; river crossings and mine warfare was then, and still is, the province of the RE.
I have many, many examples of RE soldiers with 5 digit numbers.
By and large, officers serving during WW2 had 6 digit numbers.
Hope this is useful.