Exhaust Manifold Casting Numbers

I have the original exhaust manifolds that supposedly came of my '72 duster 340. When I bought the car, they were in the trunk. They were replaced with headers. My question concerns the casting numbers on my particular manifolds. According to the research I have done the casting numbers are as follows: #3614368 = 340 '69-'70 dart, Left and #3148623 = 340 '71-'73 dart, Right. My manifolds indicate a production date with the casting number. They are as follows: #3614368, production date 2.29.72 and #3148623, production date 2.16.72. How could I have manifold #3614368 for a '69-'70 340 dart dated 2.29.72? The other manifold #3148623 (right) correlates to it's production date. I just want to find out whether or not these manifolds really came off my car. Thanks for your help.