GM cowl induction hood versus Aftermarket

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I have a LA built 1969 SS RS Camaro built in October 1968. It has a complete cowl hood set up but I do not think it was original to the car because of the build date. I was going to modify the hood to fit my 16 inch air cleaner but before I do that I want to make sure it is not an original GM part. How do you tell the differences of a GM versus a after market cowl induction hood? In case I am wrong is there a way to tell if the car is an original cowl induction hood car?
Mark Sheppard