[JoeC]

Was the M code a special order car?
In general, there were 3 ways to build factory hi performance cars. 1) regular production order, 2) special production order, 3) off site vendor production.
The Hemi Dart/Bcuda, SS AMC, 429 Mustang and others were done by outside vendors. The ZL1/L88 Corvette, LS6 Chevelle, L72 passenger, L89, L78, Z/28, and others were very hi po cars but still done RPO. What is unique about the 427 COPOs, and LT1 Nova is that they were built by special order production in-house. Almost like an undercover operation with no special ID (not even SS), and little dealer info. Super Stock drag cars coming down the line hidden among the 6cly grandma cars. In my opinion, Chevy probably only wanted to make about 50 of each to satisfy NHRA Super Stock rules. Along came Don Yenko who said "I don't want 50 L72s .....I want 500...I can slap on some stripes and sell them as an sYc"
I don't know a lot about the M codes but I think they were regular production not done as secretive as COPOs.