It took some checking and we found the M-5digit blocks were cast with no production date and many were the variety that had the 440 mounting ears that were removed.Check your block for a flat machined area above the oilpump mounting surface and machined areas where the 440 ears were.The quality of these blocks is unknown.Look also for porousity in the casting.These blocks were lighter than most Mega blocks and the coreplug holes are shallow.We found that 4 of these blocks have gone through the shop over the years,3 were able to check out and were bored 4.500 and were built to 528" street engines,one is still here.Goes to show we still can learn things if we take time to investigate.