That is an early form of shielding. It was used in very early trucks to suppress RF interference. The CCKW, AFKWX, DUKW and Chevy 1.5 ton truck all at one point had these tin shields.
They are very rare as they are very flimsy and easily damaged.
Here is the tin shield on the G506:
Here is it on the DUKW (well, best pic I have):
Here is the SNL photo of the CCKW/AFKWX shielding:
Supposed to be on CCKWs after chassis number 394577. That would be 1944-ish.
My copy of TM9-809 AFK WX-353 27 May 43 shows no such shielding.
I don't think I have ever seen a CCKW with the shielding, or a picture of one.