RHA batteries were uniquely lettered. RA batteries were uniquely numbered, by type, ie field, medium, heavy, anti-tank, HAA, LAA, survey, coast. That said there were a few lettered batteries outside RHA. For fire control and related purposes field branch batteries (ie Field, Medium, etc) of any type were designated P, Q, R & S in each regiment. Survey batteries were all in the 4n series. Why these letters I hear you ask, obvious really they are between O and T, O being the observer, T being the target.