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

By the time November 1971 came along, they were full swing into the 1972 car builds so yes this is normal. I don't know the exact date (someone else on FABO probably does though). August / September is when you can expect next years car models to hit the showroom. If this engine was in a 1971 I would say it was a transplant.

Someone once told me the factories shut down for awhile in July to re-tool for the next years models???