A couple of us have had a look at this and our conclusions were much the same I think.
The format is definately unusual, and some of the characters are not in the order that we would expect. But there are a few codes seem to be likely.
"M" would relate to an engine manufactured in Mexico.
0615 would probably signify the build date, ie 15th June.
"I" could possibly in fact be a 1, which might imply Chevrolet. (This would normally be stamped after the M)
VR signifies a '69 350 C-20 & 30 Truck Engine, 255 HP, with 4 Barrel carb and, 3 Speed.
My thinking therefore is that the "9" signifies the year of manufacture, 1969.
It's not conclusive, but I suspect pretty close! I decoded the numbers on the diff of our '78. They identified the gearing and date and place of manufacture, but there remained one unidentified character that detailed searches and conversations with NCRS experts failed to resolve, so these things do not always conform to what the books say!