I believe I stumbled upon a '68 or '69 Belvedere...

R-Belvedere, M- Medium class , 23 2dr HT, E63 - 383-4v 335HP , D21 HD 4 speed
Colour F5 F5 Lime Light Green metallic (upper and lower ) same colour as my 69 Cuda.
Just my two cents.
Mopar trim tag are read lower left to right them up.

Your tag starts with the engine code first E63 then trans mission code D21 then model RM23 next 3 digits indicate to the the engine year and plant yours should be H for 383, 9 for 1969 , plant code are
A - Lynch Road, Michigan

B - Hamtramck, Michigan

C - Jefferson, Michigan
E - Los Angeles, California

F - Newark, Delaware


D - Belvedere, Illinois


G - St. Louis, Missouri

R - Windsor, Ontario, Canada

then the last 6 digits would be you the last 6 of you serial number
You missing numbers on your other trim levels seat code should be 4 digits but you only have 3 H2T which I'm thinking is for bench seat High trim level tan colour but just guessing on that one.