Except for one (GCELW) the aircraft used by J2 for night freight were QC freighters. The seats were taken out in the evening and replaced for passenger flying in the morning. They had roller floors and used freight containers which were lower than normal to fit under the overhead lockers which remained in place during the role change.
the only loose mail bags were loaded in the belly holds if required. LW was a pure freighter which used standard containers.