"...U stamped..." Indicates Underwood. Everybody, his brother and all their cousins made Carbine parts and Mags. However, most of the original manufacturers didn't make all the parts. Rock-Ola made 13 including the receiver. So there's no such thing as an "all original" Carbine.
Go here for everything you ever wanted to know. The U.S. Caliber .30 Carbines
"...have to have all the accessories..." Yep. And that stuff has increased in value at a far greater percentage than the firearms.