It's RPO E81 and was available in 1973 - 1980 longbed fleetsides.
They weren't made using leftover 67-72 style wood floor assemblies as pig rig speculated. The 73-80 design wood floor is significantly different (has different plank widths, different cross sills, and special flange pieces to mate the wood to the bedsides/wheeltubs).
And they weren't just in the low end trucks either. My dad's 77 Sierra Classic (GMC version of the Silverado) has this option:
Before:
After restoration:
My grandfather was the original owner and he specified the wood floor when he ordered this truck. Based on his experiences with other trucks he'd owned, he found the wood floors to be quieter. Didn't make as much of a tinny/drumming noise when going over rough roads as the steel floors.