Mesh screen for a section of the belly pan...normal?

[Emerson77]

I just recently purchased a '75 Trade Wind. The other day I was under it trying to determine if my axles are bad, and while under there I noticed a section of the belly pan was a metal mesh screen riveted to the solid panels around it. Is this normal? It is directly under the refrigerator just to the left of the door (looking at the door from the outside) and to the right of the axles. I can see the bottom of the plywood sub floor which doesn't seem right to me that the untreated plywood would be so clearly exposed to outside elements. The mesh section is maybe 1' x 2' but I'm just guessing by what I can remember seeing. The good news is that the plywood floor appears to be in good shape from what I can see!

Thanks,
Eric