The possibility of a custom made carbine exists, but 20" was not a standard length, and I suspect yours has been cut down. The Army bought only a few, but the Navy purchased 250 for testing. The U.S. Interior Department bought several hundred for arming Indian police; those have U.S.I.D. markings.
The military marked guns are most valuable, but any collector value will have been considerably reduced by the barrel being shortened. .45-70 was the most common caliber but a few were made in .43 (Mauser) and .40, possibly the .40-70 Remington.
A rare and highly desireable rifle. If yours is in good condition, it would be of interest to those studying rifles of that era, regardless of the barrel. I hesitate to guess a value, sight unseen.
Jim