Both Italian repros and 3rd gen Signature Series Colts will confuse the picture. Signature Colt Navies have large squareback guards, but original Colts had small squareback guards, and the first round guards were also small. (The above chart supports this)The later larger round guard also allowed a longer trigger (or the small guard required a shorter trigger, your call.)
The second gen Cot Navies had proper small squareback guards. The 2nd gen Colts were pretty authentic to the type and models, the 3rd gen Signature Series were not, having features not used by Colt as well as missing features that should have been there, including improper stamped lettering sizes, lack or presence of cap guides, the shape of the ball cut in the barrel lug, it goes on.. The "silver" on the 3rd gen guns looks more to be nickel than silver, as well.
The Italians have added models that never existed and other changes that muddy the waters as to what was authentic.
Fingersmcgee's mention of the good book on the 51 Navies is worth finding to clarify things. Author is Nathan L. Swayze,, the title is "'51 Colt Navies", published by Gun Hill Publishing Co., Yazoo City, Mississippi, (in 1967?) and reprinted by THE GUN ROOM PRESS, Highland Park, N.J. 08904 in 1993.