You're right about the date decode and DMI stands for the manufacturer,Donnelly Mirror.The number your relaying is actually cast into the inside base,but Chevrolet recognized the entire assembly as #3965921.
It should have a convexed pivot cone like the one below,(Before 1970 they were concaved).
It was used as the Lefthanded standard door mirror and also the optional Accessory Righthanded door mirror too.
With your date it could be considered a later 1970-71 service application,as well as a 1972 Assembly line part used on:
*1970 Passenger series
*1970-71 Novas
*1970 Nomad
*1970-72 Chevelles,but NOT used on the Chevelle based Station Wagons or Elcamino either.
