1971 Dodge Dart Swinger “Special”

As Dartnut said above, the 8 1/4 was only available in 73 and later A bodies. Prior to 73, the two rear ends available in the A body was the 7 1/4 and 8 3/4. From 73 onwards, it was the 7 1/4 and 8 1/4.

Nothing unusual, weird or rare about a 318 Dart, Duster, Demon, Scamp or Valiant with a 318 and 8 3/4 between 70 and 72.