My money is on SS Perida
, not only because that is what your grandfather's IDL certificate apparently specifies, but because she was still a troopship at that time, rather well known for service in the Pacific Theater. Troopships were of course the vessels of choice for bringing home men (and women) about to be separated from the service, including former prisoners of war in a sufficient state of health that a hospital ship was not the more appropriate means.
Previously owned and operated by the Danish Maersk shipping line, German-built Marchen Maersk
took refuge in an American port (likely New York) upon the German occupation of Denmark, and with a lot of other foreign-owned vessels was eventually acquired by the U.S. under the June 1941 Ship Requisition Act. Legality and the potentially conflicting Neutality Act required some smoke and mirrors, and Marchen Maersk
became for a short period SS Caldera
under Panamanian registry. The declaration of war by the United States not longer required such artifice, and in early 1942 she received yet another name as SS Perida
, very likely at that point undergoing modification to a troopship. As far as I can determine, all or most of her service took place in the Pacific. For more details, see
It's quite safe to rule out escort carrier USS Perdido
(CVE 47), because she was still serving in the Royal Navy as HMS Trouncer
(D85) until early March 1946, when she was returned to the U.S. Navy.
Because you've raised the subject, I'm curious during which campaign Out East had your grandfather been captured? Hong Kong? Singapore? Java? Burma? How long had he been out there?