'Aerodrome' is certainly the correct term. Which would indicate someone with some aeronautical knowledge - the genpub would probably use words such as 'airfield' or 'airport'. Also 'the Collection' (with a capital 'C') indicates to me that someone 'in the know' wrote the letters.
This whole thing has a very unusual ring to it. Why did T H-T send letters to every SVAS member including police advice as to the likely source? Has there been some 'falling out' at Old Warden - and have we been told everything?
Whatever, I hope that this idiot is caught soon.