The RM21 is correct for a Roadrunner Coupe.
It could be a Belvedere deck lid.
I had to use a Belvedere deck lid while restoring my 69 Plymouth Roadrunner. It had a "PLYMOUTH" emblem on it that had to be filled and re-drilled for the correct "road runner" emblems.

Did the cars that were imported for Europe, Australia, and even Canada have that decklid? Doubt it...but that's all I can think of.

Odds are that it was a replacement from another B-body Plymouth.