YW registration numbers were issued by London County Council
This would probably mean that it went into some Government run department
As an example XO (London CC) was issued to NFS wartime vehicles in much the same way that XF was issued to post war AFS vehicles
The sequence (very simply) of Army registartions were
Pre War CivilianReg numbers some from ME series again Middlesex
From late 1930,s a cencus number prefixed with a letter denoting th eclass of vehicle T for Tank or carrier, H for Tractor and so on
From 1949 the two number two letters two numbers sequence The sequence XA-ZY were used to denote wartime vehicles, the sequence RA-RZ (excluding RN) were used to denote rebuilt wartime vehicles still serving