What letters preceded the name on the Royal Yacht Britannia
HMY HMS/M HMVS HMSm His Majesty's Ship H.M.Y. HMSAS Her Majesty's Ship HSwMS Her Britannic Majesty's Ship His Britannic Majesty's Ship
h m y his majesty s ship hmsm hmvs hmsas her majesty s ship hms m hswms her britannic majesty s ship hmy his britannic majesty s ship
h m y
WikipediaEntity
H.M.Y.