Some Merchant Navy seamen's records 1913-1972 are now indexed in the National Archives catalogue.
"A series of seamen's pouches (BT 372 ) were created for some seamen. These records relate to individual seamen. His/her papers were filed together in paper envelopes known as "pouches". The contents vary for single registry cards to ID Cards, photographs, letters, applications forms and other ephemra. The records cover the period 1913 to 1972, though few have contents dating further back than the 1930s"They are indexed by surname and initials (ie. smith r j), date of birth (02/02/1922), place of birth, discharge number
http://www.nationalarchives.gov.uk/catalogue/search.aspSearch with series code BT 372
http://www.nationalarchives.gov.uk/catalogue/RdLeaflet.asp?sLeafletID=128Casalguidi
