Frank,
I have information on many cars with that same information on their Data Plates.
J54 and N96: I think that the dual codes were implemented on or about December 16, 1969. Somewhere in between 0B220059 (only N96) and 0B220065 (both J54 and N96).
There is a point in time when that goes away again and only one or the other hood code is displayed.

Ola

6826122-J54N96.jpg (153 downloads)

1970 `cuda340 convertible 4-speed Rallye Red with Red interior
cudaized.com

1970-71 Plymouth and Dodge E-Body Convertible Registry
ebodyconvertibleregistry.com