71 block in a 72 duster. Did this happen at the assembly plant

So my Pop's and I are going over a couple of projects for the car and I elected to pull the current motor and take the original which was pulled in the mid 80's and put it back in the car until next year. For whatever reason he looked at the date on the block and it says 1971. He is about to freak out lol... I can't get down there to check things out due to my work schedule however who has had this happen? He said the block has 11/71 on it.. Any info is appreciated...

Thanks,
JW