Colt,
You have an eagle button made after the Civil War "fashioned" Navy style but most likely had a steel back (that has rusted away) and was probably made as early as the late 1800s but as the Navy motiff fashioned civilian buttons are produced to this day and with the absence of any back marks on this one its difficult to pin down exactly how old this button is. I'd venture a guess of about 50-100 years old but its a wide open guess.
A few other threads, some pretty recent, on TNET give you further perspective...
http://www.treasurenet.com/forums/what/474000-button-help.html#post4656821
Some more info on shanks ( multiple styles, including the steel type back)
Shanks
An older but very informative TNET thread on the same subject...
http://www.treasurenet.com/forums/what/304632-us-navy-button-left-facing-eagle-date.html
Hope this helps