Without seeing more pictures, it might be a factory replacement part. Our understanding is that they often didn't stamp the serial number on replacement stocks and buttplates.

Just a guess though. Even getting a letter from the Savage historian may not clear it up since work orders may not be available or found for repair work that was years later.

The lower rifle here is an 1899B from 1906 with correct forearm and buttstock/buttplate with just a code stamped on it. The buttplate is a 1920's style, so it's probably a later factory repair.

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