The most difficult time period for attribution is the period between 1944 and 1981 during which both types of reverses (four rivets and flat reverse) were in use (simultaneously? successively? flat/ four rivets/flat?). But at least today we know for a fact when flat reverse was replaced by two screws reverse. It happened between May 29, 1981 and April 25, 1983 (actually I think that change took place during 1982, but I don’t have authentic documented group to prove it).
Check this out.
Orders of sacred treasure 4th Class that was awarded to Daidou Mikio on May 29, 1981.