"Nominal" 16 gauge size is .662". Just as a wild guess I seriously doubt someone Shrunk these bores by 0.010", so likely original. Chokes would be 0.017" so also could easily be original. Offhand I do not recall if Ithaca normally used a conical/parallel or a taper choke. Perhaps someone will chime in on that & you can check the choke configuration on that basis.
"Standard" chamber length for the 16 at the time this gun was built would have been 2 9/16". A Winchester catalog from the early 1900s showed shells avai8lable for the 16 in lengths of; 2˝", 2 9/16", 2 5/8", 2 3/4", 2 7/8" & 3". Some of these lengths were dropped during WWI I believe, but likely the 2 3/4" remained available, so Ithaca would likely have made a gun in this length on special order. It may or may not be original. "IF" it has a long forcing cone that is not apt to be original.
I have a pre-1915 Lefever H grade with 2 3/4" chambers, no indication they are not original. According to L C Smith Plans & Specifications, their early 16 gauge guns had bores of 0.650". All other gauges were listed with nominal bores, I have no idea why they chose to bore 16 gauge tubes undersize.
