It is legit, somebody prob drove it like an animal when it was new and spit the rods out of it, viola instant warranty block.
I worked at a chrysler plymouth dealer in 1986. We did 3 warranty blocks in the year i worked there. One being a police packaged fury patrol car purchased by a local medical center for parking lot patrol duty. They certainly didnt need a certified calibrated speedo or a 360 for policing a parking lot. It came in on a flatbed with 4 bald eagle GT+4 tires, and an oil slick under it. They blew it up, and a rod was sticking out of the pan lol.
Point is this **** is pretty common. I bet even more so in the muscle car era.