Not all of the record is shown, but the ‘Remarks’ show that he was marked ‘R’ = Run on 29 April 1912 – ‘run’ is the naval terminology for deserted.
N.L. = I confess I have not found what it stands for, but I believe that it is part of a letter reference; it indicates the Admiralty department that originated the letter, with a serial number and a date.
So, as shown on the record, NL 4321 of 28/10/13 is just the citation for the entry, which says ‘Convicted by CP for manslaughter and sentenced to 10 years penal servitude. Not claimed for further service in RN’
CP = civil powers
Likewise, the next entry sc(?) 2252/39 Trace to NL for Scunthorpe Police would be an internal reference acting as a citation for the entry, with a subsequent date and NL number is just a letter reference.
Presumably this means that for some reason, in 1939 Scunthorpe police made an enquiry with the Navy about him, but as the Navy had no interest in him after 1913, I doubt that they could have had anything more than his service record. I wouldn't know where to start looking for that correspondence; if it survives, then perhaps in the mass of Admiralty correspondence now in the National Archives, but I would suggest it would be rather like looking for a needle in a haystack...
Incidentally, there was widespread newspaper coverage of the arrest and subsequent trial of Kent, including a photograph of him...