I think the reinforcers are from the factory, my 46 GMC COE 2t has them and I have seen the exact siffeners on other trucks same vintage. I would guess that if they were added aftermarket they would have been welded on, mine are riveted. Rusted Nuts, I looked at your pics and you don't have them. Is yours a 2t or 1.5t

Jason
1946 GMC COE
1967 Pontiac Lemans Convertible

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