Can't answer your question William - wish I could. My only involvement with aluminum castings was to help round up cores to ship out to Lockport or Winters. Only Chevrolet engineering could make part number changes, so there had to be a good reason. At such low volume, perhaps one pattern went to Lockport and slightly later version to Winters? Engineering required a foundry mark on castings to trace back the source. Maybe somewhere there is a Lockport or Winters logo on the castings? Since Chevrolet engineering made the change, it could have been just a subtle increase in wall thickness to improve strength in a critical area - just guessing.
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