Thanks! So, it seems that my man was in the Merchant Navy, as regards rating. He joined the RNR on board a minesweeper, which had been a ferry paddle steamer until the Admiralty took it over for minesweeper duties. It's a bit confusing as his very first rating, according to his service record was "Boy Sig", which I believe was Boy Signaller and a Royal Navy rating. There's, obviously, a lot I don't understand!