IIRC all of the Flues and later guns were 2 3/4 chambered from the factory. That said, I had a 1935 New Ithaca Field Grade that must have had short forcing cones because it was the hardest kicking 7 3/4 lb gun I have ever shot. Traded it off before I got into measuring tools like Skeets bore gauges and Walker chamber gauges, so I dont really know what it measured. A beautiful Grade 4 you have there.