You may be able to obtain your father-in-law's complete military service record, which may detail his assignments and whereabouts during the war, by contacting the U.S. National Archives and Records Administration (NARA). Please see this page from the Armed Guard / Merchant Marinewebsite that I manage: http://armed-guard.com/searchmil.html, in particular section II.A.1. - Records of Individuals - U.S. Military. You will have to contact the National Personnel Records Center, a NARA facility in St. Louis, MO. Provide as much identifying information as you have. There may be a charge for research time, photocopying and mailing, but the Records Center staff will provide an estimate on the cost before beginning work. It would be best if the request came from your wife, since she is next of kin to her father (next of kin=parent, spouse, child, sibling). Next of kin will be able to obtain more information than someone not next of kin. The number on the seaman's bag may be his military serial number (ID number) and may be a useful bit of information in contacting the Records Center. From what I have seen, officers' serial numbers had six digits.
Ron Carlson, Webmaster
Armed Guard / Merchant Marine website