I'm just not sure since all the "original" seats other people post seem to be the 2nd layer. I haven't been able to find any other pictures like it. It's GMC also and I can't find any old brochures for GMC options.
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.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)