Through the World ships naval forum I have so far acquired the following, if I learn any more I will keep you posted.
I believe that T only refers to "borne for training only and not assessed".
It usually refers to short periods of service on any ship or shore base, where his Divisional Officer would not get to know him sufficiently to remark on his Character and Efficiency.
Book closed as per AO, could mean his service was curtailed as per Admiralty Order as he had only completed around 4 years of service and was still an Ordinary Seaman, when he had signed on for 10 years. There is only half of the page to this record and there may be an entry at the bottom of it which would give an explanation for his premature discharge.
It may be worth posting this record in the main Forum, where someone may be able to add further comments.
Jim
Regards, Bob.