I wonder how many of those were built? Or where they came from? Were they service replacements, rebuilt and diverted to the MP program, Junkyard like you said? Picked off the line? There's no 7 digit casting number -dash CID on the driver side or casting date on the passenger side in line with one just above the freeze plugs? I remember the MP catalog at least at one time described them as 20-over and "seasoned blocks" which most likely says used then rebuilt/remanned. Maybe they weren't all that way?