On the engineering drawings -
0 = all
**** so the "MODEL" line would read G/R-0-23 1971/Plymouth B-body-ALL-2 door hardtop RM23 (Road Runner) & RS23 (GTX)

$ = extra-cost OPTION

I also agree that the drawing & Plymouth's "intent" was to have the vent area of the "AIR GRABBER" hood (behind the insert) painted with "organisol". The issue that is clouding the topic is V21 or "our" interpretation of what it is.

1 - I can find no evidence of V21 being an orderable OPTION (with or without cost)
2 - It is included with OPTION A87 $85.60 on Road Runners - "Road Runner Decor package"
3 - Road Runners & GTXs were assembled at 3 different plants in '71 - coding differed from plant-to-plant & during the year