Used the decoder over at MyMopar and came up with the following...
http://www.mymopar.com/index.php?pid=87
RM21:
Plymouth Belvedere,Satellite
Medium, Road Runner
2 Door Coupe
G4G: 318 150HP(net) or 170HP(net) 1-2BBL 8 CYL
1974
St. Louis, MO, USA
202678: Sequence number
E44: 318 cid 2 barrel V8 230hp
D34: Light Duty Automatic Transmission
EW1: Eggshell White Exterior Color
D6X9: Trim - Luxury, Vinyl Bucket Seats, Black
000: Full Door Panels
301: Build Date: March 01
228508: Order number
EW1: Eggshell White Top Color
U: USA Specifications
A88: Interior Decor Group
B43: Manual Brakes
C16: Console w/Woodgrain Panel
C56: Bucket Seats
G54: Outside LH Remote Chrome Racing Mirror
G64: Outside RH Manual Chrome Racing Mirror
G74: Dual Chrome Racing Mirrors
J54: Sport Hood
M21: Roof drip rail moldings
M26: Wheel Lip Moldings
N41: Dual Exhaust
N42: Chrome Exhaust Tips
R11: Radio Solid State AM (2 Watts)
V8R: Tranverse Stripes, Red
V9R: Unknown