There are several scenarios as to why and how. Bottom line is the original is gone and there is a different one that replaced it. Could be a warranty engine, junk yard replacement, an rebuilt-er replacement when the original was tired or windowed. Remember these were cheap cars and not worth spit until 15 years ago. Stock to stock the 69 is a better 340 than the 72. Either are good to rebuild. I've even heard of people pulling motors and selling cars and the buyer had an engine they put in it, drove it and sold it. Screwing with a car was much easier back then.
