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 !!