Reply to Marvin re: Springfield-Remington Carbine...
You do, indeed, have a special carbine. As you may be aware, the National Armory at Springfield, Massachusetts manufactured 10,000 Remington pattern rolling block rifles in .50-70 Govt for the U.S. Navy Bureau of Ordnance in 1870-71. It was stated that the rear sight was placed too near the breech and was "unsafe", so the entire lot of 10,000 new rifles were sold to the Baltimore firm of Poultney & Trimble (who fronted for E. Remington & Sons in arming the French during the Franco-Prussian War of 1870-'71).
A bitter Board of Inquiry looked into this matter, stating that General A.B. Dyer, Chief of Ordnance, was in colusion with the Remingtons to supply firearms to France... a problem since the U.S. Government was supposed to be neutral during the conflict.
Poultney & Trimble paid the Ordnance Department enough for the 10,000 rifles for the Armory to make 14,000 new rolling block rifles (with the "correct" sight placement) for the U.S. Navy.
The Navy got 4,000 more rifles than they originally paid for, and General Dyer's workmen at Springfield got thousands of hours of work.
After this long-winded history.... a few words about your carbine. About ten years ago I had the honor of spending a week at the Springfield Armory examining and photographing the dozens of production and prototype rolling blocks in the collection (managed by the U.S. Park Service).
Among the arms were several carbines made from cut-down M1870 Navy rifles (such as yours). Some had the original rifle barrels shortened (the muzzle diameters were excessive) and others had new carbine profile barrels. The "carbines" at Springfield, and yours, are honest guns. Not gunsmith conversions.
The Pattern Room you mentioned was (as Ed Hull stated) at the National Armory in Springfield, not at E. Remington & Son's Armory in Ilion, New York. Remington licensed the U.S. Government to make these rifles... and 100% of the work was done in Springfield, with a royalty paid the Remingtons.
I would welcome photographs of the presentation plate. They sound appropriate for my upcoming book on "Remington Rolling Block Rifles, Carbines & Shotguns".
Please send to me at: firstname.lastname@example.org
Glad to help,