its called a lower radiator baffle. It keeps the dirt and weeds from being sucked up into the radiator core. It also support the bottom of the grill. On your truck it would have had a hole in it that a crank would have gone though, if one was man enough to try and hand start the engine. On the big trucks there hard to find as most rusted out in the center where the grill support attachs. Like the upper baffle, they will only fit a big truck. The smaller ones use a different size and bolt pattern on theres.

Jeff
1947 Chevy 6400 2-ton
In the Gallery
1953 3/4-ton
In the Gallery
It was fun while it lasted