Interesting tags Tom_Q - thanks for posting!
I have heard - not gospel, that pre-production and first day vehicles were typically Pilot cars...to do dry runs down the line for fit and finish and possibly to supply pictures for parts books, manuals and even promo photo's. I imagine promo/show cars came next.

I saw that EL5 71 Challenger that Gregg brought to MCACN last year. It did have a lot of 70 stuff and some weird stuff on it....very interesting....although it was restored. Hopefully it was fully documented and photographed prior to the resto.

The "M" special order numbers are probably relevant...as well as the sequence number on the broadcast sheet. It would be interesting to be able to sample a good batch of this information.