Here's your codes again;
M43 Grille Trim Mldgs
N41 Dual Exhaust
N94 ESA w/catalyst
R31 Single RR Seat Speaker
J25 Deluxe Windshield Wiper
J52 Hood Release - Pnl Mtd
J88 Rear Aerodynamic Spoiler
K4W Stripe Type 4 - Silver/White
L31 Turn Sig Lite - Hood/Fender
C16 Console
C56 Bucket Seats, Non-Recliner
G11 Tinted Glass, All
G15 Tinted Windshield
G77 Dual Mirrors - R/PNT RAC (Remote, Painted, Racing)
H51 Air Condtg W/Ft Htr
MV1 Spitfire Orange
U5 ?
A57 Road Runner / RT PKG
C15 Spl Sound Insul Type 1
L6X9 Bucket Seat - Cent. Cush Arm Rest - Vinyl
E58 360 8CYL 4bbl Dual Exh
D35 HD Auto Trans
105 Scheduled Prod Date January 5, 1979
L1 End of Sales Codes
VIN Info: NL29L9B203720
N Dodge Aspen
L Low Price Class
29 2Dr Special Coupe
L 360 1-4bbl HP
9 1979 Model Year
B Hamtramck, MI Assembly Plant
203720 Sequence Build Number
I think the long narrow tag was stamped with numbers by inspectors as the car moved down the line.
My '79 road runner has the same scheduled assembly date, at the same plant as your R/T
In 1979 there were a total of 1,137 Dodge Aspens, and Plymouth Volares built with the A57 option. Aspen R/T, Plymouth Road Runner.
Looking under the seat cushions or the seat back cushion, you may find the build sheet with all the build codes on it. The back of the sheet breaks down the codes. It could also be stuffed up into the dash somewhere, or under the carpet. Good Luck.
I also found out the original selling dealer changed hands in 2017, and it is under a new name, at the same location apparently. If its not too far away, you may want to get a picture of your car in front of the dealership, or look for advertising items from the dealer (Forrester in Arnold, I believe). It might be a long shot, but you could ask if there are any records at the dealership, or with the Forrester family, from the year your car was sold.
Very Sharp R/T !!