Seems wierd that it wouldn't be marked.
Open the action and roll the rotor around with your finger, most 303 Savages were marked with "303" on the back of the rotor.
Another option would be to send for a factory letter from J. Callahan.
The safest option would be to take it to a smith and have him verify the chamber as 303 Sav.
It was made in the early 1900's, the only other calibers it could be would be 25-35, 32-40, 38-55 or 30-30.
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