Tonawanda did supply some 010 blocks to the Flint motor plant for machining into 302 and HP 350 engines, but only when the Saginaw gray iron foundry couldn't keep up with production demands. The blocks from either source were interchangeable, but proximity to the engine plants dictated the preferred casting supplier. Any "special" changes that I alluded to earlier were marked with a character to identify the casting as special. I frequently used a "X" that was pressed into the sand mold of the casting. Since the inspectors in the foundry finishing department had to check the castings for a complete date code - it was convenient to put the special "mark" near the date code so that the inspectors would notice it & specials could be set aside & not mixed with regular production. Not sure what was special about your "M" coded blocks. We used an "X" every time a special was run - so it didn't always designate the same special feature. The "X" code along with the specific date could be traced back to the specifics of the special.
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