The 'C' is probably a 'G' and that's Saginaw. However, none of the prime contractors made all 60 some parts. Saginaw only made 17 of 'em. Standard Products used a plain 'S' but only made 4 of the parts including receivers. It might be Sprague & Carleton, but would not be their metal supplier. Metal suppliers just sent bar stock.
Literally hundreds of small shops all over the Eastern U.S. made parts. That alone means there's no such thing as the fabled "correct" Carbine.
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