Ask the seller for a full size in focus image of the barrel flats, then post, or send to me at drewhausemd@yahoo.com
The proof marks will tell the story, and until post-1954 had a number indicating a range of bore dimensions. It is likely the .740" was part of the reproof after honing.
3 1/4 Tons would be standard proof for 2 3/4” chambers 1954-1989 for a service load of < 10,000 psi

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