Judging from the serial number you posted and the fact it has an Underwood barrel, I'd say you have a Saginaw S'G' carbine. The first Irwin-Pederson/S'G' serial number range ran from 1,762,520 to 1,875,039. I'm guessing the barrel is dated 8-43 or 9-43?
Hope this helps . . .
PS Sorry for the redundancy, I thought I was the 3rd poster!Am I close on the barrel date?