I checked what I could in the "REFERENCE OF RUGER FIREARMS".

The Reference lists physically known models. It has ...

1 known M77ST 7X57 in a flatbolt, SN 70-45989.

No known in a Hollowbolt, but are a possibility.

And the earliest known M77ST 7X57 round bolt, SN 70-46109.

This Refference does not distinguish between 22" or 24" barrels.

I have This one, it is a 24" medium barrel.

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Ed