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

That's nothing unusual. Parts are cast before assembly and '72 car production/assembly actually started in late '71. Restorers look for date codes for parts that are earlier than the date of car assembly. But there are occasions where Mopar used leftover parts from the previous model year. So it would be interesting to see if it had the earlier high compression pistons and/or 2.02 heads instead of the lower compression & 1.88" intake valves.