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I would take the barrel/action from the stock and see if there is anything stamped on the barrel that was below the stock .
If there is nothing, look through the ejection port at the face of the barrel( where the shell goes in) and see if there is some writing or stamping there.
You might have to clean the area first.
If you don't find any writing/stamping, the next step would be to do a chamber casting to get measurements of the chamber
A gunsmith can usually do this for you. You can do it yourself also if you have the knowledge and the casting materials
You already know this but I have to say it:
I would not shoot any ammo in the rifle until I was sure of the caliber
Jack
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