a) American export Lines' SS Exemplar made her maiden voyage in Nov. 1940 - Bombay, Calcutta, Colombo. The envelope is postmarked March 4th 1941 with instructions to meet SS Exemplar March 11th 1941 at Calcutta. Perhaps second voyage ?

b) Hospital city seems to be Jhansi - googling shows more than one missionary hospital + others in that city.

Hope that helps 

shot in the dark ...  "Major R.E."  = Major, Royal Engineers, who approves / arranges ?. ... referring to Royal Engineers (postal section), British armed forces. Does this apply to BFPOs in 1955 Pakistan ?