Ithaca made those barrels themselves. According to Dave Henderson's book "Shotgunning for Deer", Ithaca had been outsourcing their barrels because the forge had been too expensive to move and reassemble during one of the companies many moves. After a few years, they (Ithaca) purchased a computerized barrel lathe in 2003 and began making not only barrels for Ithaca guns, but slug barrels for Remington 870 and 11-87's, Browning gold and Winchester super-x2 and Benelli Nova shotguns.
I purchased one new from Cabelas for a Benelli Nova and the fit, finish and quality was very good. It proved to be an accurate barrel as well. I sold the gun a couple of years later (dumb move).
I think the twist rate was 1-28. FWIW, mine shot very well with Remington Copper solid ammo.
Jim