[Steve Shauger]

Thanks Bill & Kurt, I was looking for a thread I started in 2004 which John Z. confirmed what is stated on the CRG:

"M"-Coded Blocks
There is one exception to this convention that has been observed: some blocks have been seen with "M" cast codes. Research by the Saginaw Metal Casting Operations of GM Powertrain Division has found that these "M" codes are not date codes, but actually a foundry code used to indentify parts that were made to evaluate a casting process change. After evaluating the parts, the blocks would either be destroyed or released into production. There is no way to determine the date an "M"-coded block was poured.