Because Saginaw Steering Gear at Grand Rapids MI did not make their own barrels and used what was called the government free issue program. This gave them barrels for free and reduced the over all cost of a carbine. Underwood made the most extra barrels for the program and was the one most often found on Saginaw S'G' carbines. A sister company Saginaw S.G. in Saginaw MI, but considered a different maker made their own barrels. The 8-43 is the date the barrel was made and is pretty close to the date of your carbine.