What is "M" "2" on a 67 fender tag?

I have two 67 Formula S 383 Barracuda's and both have M2 on the fender tags, but my Galen books don't call out the "M". What is it? In his example in the white book it even had an "M2" but doesn't say what it is...

Maybe means moldings deleted

I can't find it anywhere. Moldings are under other letters, but maybe included in this one too?

I read where it was not an option, but meant the car was built on assembly line 2.
Similar to later years where tags used EN2.

You are the second person that has told me that, so it must be true if a third person says it without reading this...LOL

http://chargersourceguide.com/67fendertag.html

Leave it to a Charger group to figure it out, us A-Body types just don't have the resources...LOL

1966-67 fender tag gurus