From what I know, Argent complete assembled units with nameplates intact, were sent back to Fram in bedsteads of 60 in 1969 when the run of 1970 Challengers with shakers ceased for a period of time.
Challenger shaker option resumed again in spring of 70 but when the 70 model ended there was still a bedstead or so of argent assemblies sitting at Fram.
71 model year saw Chrysler implement the change to black scooped assemblies beginning in August 70.
Sometime in the spring of 71, whether it was a shortage or just a vendor decision, Fram decided to use up existing stock of argent from that leftover bedstead.
There may have been some argent assemblies used on earlier Challengers as well but I know of quite a few 71 factory shaker Challengers with argent scoops, built in and after April 71.