Someone trying to be dishonest would be one reason grinding them off and smoothing them over. Selling a /6 car as a re-bodied factory 383.
normally the paint would cover the numbers up but if you carefully run your fingers on the other side of the panel you should be able to feel the stampings. take a piece of paper and place over the stamping rub a pencil over it sometime just you nail will work.
if they are not there look for past damaged as I mentioned, welds on the core support ripples in the metal over lap of panels etc
