Apparently your fender tag is for a 69 2 door post coupe reflected in the coding.
RM21H9A230825
RM21= Belvedere, Road Runner, Medium, 2 door coupe.
H9A=High Performance and produced at the Lynch Road Factory.
The bottom row again shows the RM21 first
E63=Engine a 383 4 barrel 335 HP
D21=Transmission
215=build date February 15
902885=Order number
Second line from the bottom
96=Exterior
H2X=Interior
Third line from bottom, I believe the sideways “G” is an identifier for this line meaning
G1X=Glass, tinting windshield
G7X=it could be G72=Out Side duel remote electric mirror or G73=Out Side duel remote manual mirrors, however, the 1X and 7X could also refer to paint color for interior representing Black. It is not a high optioned car, so the 1X or 7x could also mean, manual steering and manual brakes.
Second line from the top
925=Engine code 383 4 barrel manual trans, heater, 335 HP, Orange
595=3:23, 8 ¾ rear axle 11' rear & front drum brakes
603=Manual A833 Transmission 23 spline
Top Line
08112=not sure
237202=not sure
9A23085=Lynch Road Factory, and car serial number