I've found the following information about a spring, but I'm hoping to find more information about the entire assembly.
1973-75 GM TRUCK BENCH SEAT BACK RELEASE ROD SPRING
NOS Out of Package
Manufactured by General Motors
Part Number 335507
APPLICATIONS
73 GM Truck w/Interior Storage...
My new to me 76 K10 has the baseline bench without the folding seatback. My 77 C10 had the folding back, and I was surprised when I couldn't fold down the seatback. Anyone know what part numbers the seatback brackets/latches are, or where to find some other than lucking out at a junkyard?