Wive's tale IMO................in all of my years I have never seen or heard of more than two partial VIN's (last 8 characters of the 13 character VIN) stamped onto the radiator support and trunk drip rail (ie., body) of a '69 B-body. The only other locations for any kind of VIN identification is the engine and the transmission. If there were ever 19 stamping(s) on a '69 Mopar someone by now would have found a car with more than just the two known locations.........

As stated, as of now there's is no way to prove what it is or what it was or what it wasn't so the owner can make it what ever he wants it to be and no one can prove him right or wrong.

The only thing in my opinion that may give any kind of a clue is if the doors and trunk lid are original to the car and then (I'm not sure) but if the holes for the door "road runner" emblem and the trunk lid "road runner" emblem are still visible from the door's and trunk lid insides (like filler or weld) that's the only clue IMO that it might have been a RR???

Edit: taking another look at the '69 Plymouth B-bodies the road runner was the only B-body with an emblem on the doors and the trunk lid had no filler plate and the road runner emblem on the passenger's side. And yes a HEMI emblem on the driver's side. the Belvedere the other only coupe/post B-body had a large PLYMOUTH emblem on the passenger's side. If there's trunk lid filler plate mounting holes that were closed and visible from the inside of the trunk lid then that really throws a curve in this as there was no '69 coupe/post car that had a trunk filler plate