Larry Ruth (M1icon Carbine Owner's Guide, 2005) says Underwood produced up to approximate serial number 4046000 in September '43, at which time they stopped using the third block (4M range) serial numbers and started using second block (2M range) serial numbers, many on sub-contracted receivers. He also says they used strictly the second block numbers in Oct, Nov & Dec '43 and resumed using the third block numbers (along with the remaining second block numbers) in Jan '44. This kind of makes sense because all of the second block carbines and the later range of the third block carbines are seen with fabricated trigger housings, which may have existed in Sep '43 but may not have been in use until Oct '43. Craig Reisch says the changeover from milled to fabricated housings was at approximate serial number 4050000, which is pretty close to 4046000 when Ruth says they interrupted use of the third block (4M range) numbers until months later when the fabricated housings were then standard.

I don't have War Baby or CC newsletters, which may indicate otherwise, and the M1 Carbine Owner's Guide is getting pretty old, but original barrel dates on every Underwood carbine I have seen pretty much supports the timeline described above. FWIW, ChipS
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