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I always thought Yenko bought the molds from GM and had the blocks cast at different companies in the 70s. I thought there were different configurations available depending on bore size and block height needs. I understood some of the blocks came with the Winters snowflake with the GM part number still on the block, some had the SYC crest, some had YENKO cast in the front and some had no markings at all.
This always confused me because I also heard the story how the GM engineer found the original molds for the ZL1 blocks in the trash at a GM plant and reworked them. Then GM reintroduced the current 053 aluminum big block from the updated molds.