If you know the job ID (e.g. #J1234) you can get the spool file ID of its $STDLIST

setvar HPLASTJOB "#J1234"

setvar spoolid HPLASTSPID

Note that HPLASTJOB is writeable!

:HELP VARIABLES and :HELP variablename give, respectively, a list of all pre-defined system variables and a detailed description of any one specific variable. Unfortunately, the HELP text sometimes lags behind the implementation of various variables. Therefore, it is always a good idea to have recent Communicators available. New CI options are almost always documented in the Communicators.