Exactly what makes a MOPAR a "Matching Numbers" car?

Let me be more clear. Factory correct happens once, as it rolls off the assembly line and even some of those cars were not numbers matching or factory spec correct. You know we ran out of parts so we substituted kind of thing. You and I were not there
I ordered a new 70 Duster slant car Dec. 26 1969. Lucky me, when it arrived it had a 426 hemi....the dealer said they ran out of stants that day and there it was with my hemi!!!!!! True story! :steering: