Hey Pat. Welcome to the site I have a '71 RR, buckets, Tor-Red, A/C, AND a cassette as well. Half vinyl roof as well. No stick tho. Anyway here is what I can tell you:

"N" Is the engine size in the car, nothing more- If the block has a "HP" on the pad near the disributor and/or a symbol with it, then it is better than the standard 383. Not sure about the spcifics on the engine as to why. I think a better cam etc. Someone will chime in and clearify this better.

E65 383 4BBl magnum engine
D21- Stick shift
RM23N1G-VIN number and the rest of the numbers would be the sequence(number what ever of what ever)
GY#3"Paint color-Curious Yellow
M6X9- Interior
TX9- Black trim?
610-Build dat- Not sure when
040964 Order number I think
V4X-Not sure
U-Built to US specs-Not a export car
A01-Light package
A87-Roadrunner decor group
B11-Heavy duty drum breaks
C16-Not sure
C55-Bucket seats
G36-Not sure
H51- Air conditioning
J45-Not sure
L31-Hood mounted turn signals
M21-Bright drip moulding
N41-Dual exhaust
N97-??
R36: Thats the radio. If you have a cassette player, then the player is 1 of 137 made for that year, from what I've been told.
V21-Hood performance treatment
V8X-Strobe stripe reflective, black
26-26" radiator

The ones I don't know about someone will decode for ya. I don't have all of my numbers with me right now. Post some pictures also. If your restoring her,, progress will be appreciated. Also, become a member of www.roadrunnernest.com. Thats a site for '71-'74 B-Body cars. A lot of info there as well Congrats on the car