I have to disagree with Zonie just a bit.
A few photos would be in order: left and right side view of the frame and barrel lug. GU was Gregorelli & Uberti who made 1851 Navies for Navy Arms from ~1959-1962 before Aldo Uberti split off from Gregorelli in 1963: Gregorelli had the manufacturing license and Uberti had the factory. Aldo Uberti obtained his manufacturing license in 1963 and that is where we are today. In those days the GU guns most likely had no proof marks or date codes as they were most likely produced for Val Forgett's Navy Arms Company shipped to Val in the US and marked Navy Arms.
Don't take my word for it:
Civil War Guns by William B. Edwards (1962) pp 421-427 explains it.
There is also this courtesy of Dennis Russel:
Regards,
Jim