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The -7 is NOT a core shift number, it is a number telling how many repairs have been made to the core.
Think about it, core shift happens during the pour. Any numbers cast onto the block have to be in the mold before the pour.

??
How does one know this?
Cores are not repaired as a normal case. A corebox, used to create cores, can be repaired or altered.

The "-7" number would have to be labeled on the pattern used to make that particular casting. Probably just a designation on the pattern revision or an identifier for the particular pattern if had more than one pattern (most likely) to make the castings.