Starting to work on a 68 Road Runner, starting to look into the history of this car, tried a few online decoders with no luck, I have some codes but the rest nothing


My data plate :


A B C D E F G H J K L M N P Q R S T V W X Y Z
1 S (side ways)
1 2 3 4 5 6 7 8 A X T R M P N I U B S
5 4 8 H 4 B Q Q 1 U
RM23 62 5 46 709 369090




I know :
R - Belvedere ? maybe Plymouth ?
M - Road Runner
23 - 2dr/Hardtop
62 - 383 w/4bbl
5 - 3spd auto trans
46 - F70x14, white wall, steel belted tires
709 - build date
369090 - vin
48 - ?
H4B - ?
QQ1 - I think its " Frost Blue Poly "
U - ?


Thanks for any help

 

369etc is the vehicle order number, vin isn't on a 1968 fender tag