I did a check of the barrel date / make found on CMPicon auction IP and early S'G' carbines that were sold on auction for the past few years. The 3.21 IP and S'G' receivers most commonly had Underwood barrels. The most common barrel date / make was 4-43 Underwood followed by 3-43 Underwood. There were a few 6-43 Underwoods (must have finished later). There was one 2-43 Underwood. A few Inland barrels showed up also ( 3-43, 4-43, 6-43 ). There was one 8-43 and a few so late they had to be replacement barrels.

Hope that helps.