I don't know ! But in Britain we only have 2nd Lieutenants and Lieutenants in the Army ,but no "First Lieutenants" as such.That is an administrative position,rather than a rank,in the Navy,where,on a very small vessel,it could conceivably be a Sub-Lieutenant,or indeed,on an aircraft carrier,a Lieutenant-Commander,as it was on /in the carrier I served aboard for three years,along with at least one other " 'Catter" that I know of !