1969 Wood Bed Cross Sill Question

[mi396ss]

I was looking in LMC Truck and they show a 49.5" reinforcing strip that looks like it goes between the wheel wells under the bed. My truck didn't have this from the factory nor does some of the pictures I've looked at that people have posted of their wood beds. Am I missing this piece or is this a piece they've added for strength? The smaller (1" high) cross sill that I have is the same length as the others and goes at the front of the wheel wells. Can anyone clarify this for me? Thanks.