My thanks to wieesso and NewForest for the very useful info on the Sealand!
What I can still cannot figure out is why the apparent system of naming production aircraft as Sealand 1A though 1N was used (The Sealand 1 was the prototype?). Another puzzle to me is that the pure flying boat Sealand 3 (S.B.7) is sometimes also referred to as the Sealand 1M

Thanks again!
Wout