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

Some mighty big stories there, pics or (as they say) it didn't happen.
I hate to tell you but I don't have pics of cars I had 3 year ago, much less 40 years ago!!!!!
But I always said... believe what I tell you, I don't have to make yup stories..real life is strange enough!!!!!

Back in the 90s (again) I had a car bud and his brother, lived in a very rural county. About 3 hours away. They knew every Mopar car guy in that county and surrounding counties and everyone knew them. The one brother was always buying some old Mopar and he would play with it a short while and many time would call me to come get it (as in BUY it) (but always bring my own wheels!). He calls one day and has a 68 or 9 Roadrunner. It even had the # motor/trans but he knew the ca from the day it was new and knew everyone that had owned it since then. He said he knew for a fact that car had at least a dozen different 383s (maybe a few 440s) put in it, but as luck would have it, the last guy got hold of the original engine and rebuilt that! That was about 2000.