Well, yes, it's true that the 21st Army Tank brigade consisted of 3 regiments each of which was a 'RTR' (royal tank regiment). I had not heard of this formation before by the way, I had to google it. I'm not really an expert. I think the logic goes something like this: The King doesn't want to be associated with the PBI (poor bl**dy infantry) and so doesn't give the Army as a whole the right to call themselves 'Royal'. Only selected units get to be Royal, the rest of them are the scum of the earth and really shouldn't be associated with Royalty. The tankers are amongst the exceptions he is willing to make.