A silver eagle is the insignia of rank for Navy and Coast Guard Captains and Marine Corps Colonels, which are in the O-6 pay grade. Captain is the highest Senior Officer rank in the seagoing services, surpassed only by Flag Officers (i.e., variously ranked Admirals), while a Colonel is the highest-ranked Field-Grade Officer in the Marines (as well as Army and Air Force) and is outranked only by General Officers (Brigadier General, Major General, etc.).
Eagles were first used as rank insignia for Captains in the Navy in 1852 on epaulettes and shoulder straps, but the insignia had a maritime flair in that the eagle was perched on an anchor. Nearly 90 years would elapse before a metal pin-on version of the Captain’s eagle appeared in 1941, but the new version had eliminated the anchor and instead used the same design as the eagles worn by Colonels in the Marine Corps and Army (copied by Air Force in 1947). Use of the eagle as rank devices on epaulets or shoulder straps in those branches dates back to about 1831.
In the Navy and Coast Guard, coat rank devices are worn on the shoulder straps or epaulets of outerwear garments such as the All-Weather Coat and Blue and Black Jackets (Navy) and the Trench Coat and Windbreaker (Coast Guard). The insignia centered between the front and rear edges of the strap 3/4 of an inch from the shoulder seam, with the device oriented so that the top of the eagle’s head is pointed toward the collar and both the eagle’s face and the olive leaves are pointing forward. This same configuration is employed by the Marine Corps in its regulations regarding the placement of the Colonel’s eagle on Dress jackets, Service/Dress coats, the Tanker Jacket, the All-Weather Coat, and the Field Coat.