Maybe pilot's original flight was cancelled and got a SBY boarding pass for the 7am flight next-day. After check-in time, they found one J-cabin passenger did not check-in so the space is opened for those who are on Full-J SBY list. This will make sense that the pilot shows up with a SBY boarding pass and ask GA if there are any seat. In either case, GA will issue a new boarding pass for 7am flight with J-seat or SBY for next flight.

SBY = Stand-BY