Originally Posted by
CAEBr
The letter prefixes were introduced in 1916, with the Axxxx batch allocated to RFC aircraft, and Nxxxx allocated for RNAS aircraft. It continued to be used after the RAF was formed, for Coastal/Fleet types.
After 1926 they went to Sxxxx, S standing for Sea. The R series was already in use, so S was both fortuitous and the next available.
The S series was relatively short-lived and was abandoned in 1930 after less than 10% of the range had been used, with subsequent nautical aircraft being serialled in the regular (then) K series.
There was of course a second N series (following on from L) with no specific naval connections.