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.