The No and Grade of B.T.Certificate simply means the Number and grade of his qualification as issued by the Board of Trade. He qualified as a 2nd Mate in April 1914, Certificate No. 008409, but was obviously sailing as a 3rd Mate at this particular time. I do not know what the other number stands for but it clearly says NHS Insurance Number which I would have taken as being his National Health Serice number. The NHS only came into force in July 1948 so if you are sure it is earlier than that it must be something else. There is nothing on the document to indicate which ship it was but you might find it worth looking at "theshipslist".