Letters tend to be used for platoons within larger units such as companies or regiments, whereas numbers are often used for independent or one-off platoons. My next door neighbour did his National Service as a driver in the RASC, albeit a bit later than you grandad. He was a section commander in A platoon of the RASC Ambulance Company based in Bulford, so letters certainly were being used at that time.
I agree with you that very little appears online concerning RASC platoons of this era. However if you want to find out more about the day-to-day life of a soldier in the RASC I suggest you contact the RLC Museum:
https://www.royallogisticcorps.co.uk/museum/As I'm sure you are aware, in 1965 the RASC morphed into the Royal Corps of Transport (RCT) and some smaller corps such as the Army Catering Corps, while some of its former fiunction (stores and staff clerks for example) went to the Royal Army Ordnance Corps (RAOC), before the RCT and RAOC were recombined back to form the Royal Logistics Corps in the 1993.
They offer a research service although they do say it's unlikely they will have any information which relates to specific individuals, but they may hold more details about 17 Company, and its functions in post-war Germany. There is also a digital archive you can search online once you have registered:
https://www.rlcarchive.org/Welcome
