My research indicated that the changeover from IP marked receivers to S'G' marked receivers was before 1,791,xxx. IP marked receivers with serial numbers higher than that were stragglers in production. It appears that Irwin-Pedersen stopped stamping serial numbers before 1,791,xxx and started stamping the 3.2 million serial numbers for awhile. Around June or July, they changed back to the 1.79 million serial numbers and finished the block. I have a 1799xxx carbine with an early Buffalo Arms barrel dated 7-43. See Carbine Club newsletter 374 for club data on this subject.