Odd ball car here, need some help

[X99]

Whats up everyone. I have kind of an odd ball car here that I need some help figuring out. Its a 1970 Fastback Mustang........that has a numbers matching boss 351 cleveland motor in it. Correct me if I am wrong here, but the Boss 351s came with factory aluminum dual plane intake manifolds, huge ports on the cylinder heads, closed style chambers, and 4 - bolt mains. Also the only motors that had the VIN stamped on them were the Boss engines. Everything I have mentioned, the motor has. Then number on the motor, matches the number on the car. Is there any explanation for this? Could this possibly be a 'mule' that ford used to test the motor? I know that this motor wasnt 'officially' available until 1971, but like I said this is a '70. Anyone ever heard of something like this before, because I am stumped. If it helps out the VIN of the car is 0F02M144727. I am aware that the 'M' stands for standard cleveland motor. But like I mentioned before, the motor, if my information is correct, is a Boss 351. Any help that anyone has would be great. I would really like to figure out the story behind this car!!! Thanks in advance!!!