This seems to be your fathers Seamans Pouch.
Registry of Shipping and Seamen: Central Register of Seamen: Seamen's Records ('Pouches')
Records relating to individual seamen filed together in a pouch. These documents may include application forms (most with a photograph of the seaman), certificates, identity cards, cessation documents and notifications of death.
The contents of any particular pouch, however, may cover any period starting after 1913 and ending up to c2002. When seamen were discharged some or all of their documents (including the index cards) were placed in the pouches and these include discharges of seamen who were originally registered in the Central Index Register of 1913 to 1941.
You should be able to obtain a copy by clicking on the link "Request a Copy"
He should also have a Form CRS 10 which will give the names of the ships he sailed on. This would be the "office copy" his Discharge Book and I would not bother to obtain it. If you do wish to obtain it. Just make another post.
I have a 3 page copy of the Survivors Report for STENTOR. If you would like a copy just send you email address via a personal message. Too large a file to attach. You can also obtain from TNA Kew.
Piece ADM199/2142, pages 355 to 357.