The other point is that Lieutenant RM served eleven years in the rank before being promoted Captain RM. As this was at about the same time as Lieutenant RN would be upped to Lieutenant Commander RN the equivalence bit kicked in here.
An interesting point from the navy of old was that if an RM detachment aboard a warship was commanded by a Captain RM he would be called: 'The major' - another piece of staggeringly useless information!!!!!!!!!!!
Old Duffer