Maybe 71 was the last casting date for the 413? Mopar might cast a load of them and the supply lasted through nearly to the end? The only things the 413 was available in were big trucks and RV chassis, how many do you suppose they expected to sell with special order 413s? Some of the end production trucks and RVs came with 440s, which could indicate they finally ran out of 413 blocks and filled the remaining orders with 440s.

I know it wasn't unusual to see Dodge chassis under RVs that were 2-3 year old manufactured chassis sold under the newer RVs. It would not be out of the realm of believing Mopar used a warehouse full of old 413 castings until they were gone. Gene