I don't have any 1939 GMC reference material to confirm, but the 1939 Chevrolet Truck dealer handout description of seat constructions says:
"Drivers Seat - A single full width cushion. Underneath the durable rubberized fabric upholstery is a latex-bound hair pad on resilient springs."
2nd pic below is seat construction cut-away from same dealer handout. Looks like yours has been recovered a couple times.
