I have a friend whose g-g-grandfather enlisted in the !st Va. Cavalry as a private, and rose to a 2nd lieutenant until wounded in Nov. of 1864. She has his 1860 Ames sabre, 1851 Navy Colt revolver (4th model) & holster, Spencer repeating rifle, powder flask, two stirrups, saddlebags, and one boot cut off of his foot - he was shot in the ankle). The sabre, revolver (made in 1863), & rifle all were originally Yankee issue.
I have a .44 Remington revolver that belonged to my g-g-grandfather (a Confederate major) - he improved his inventory along the way.
I have a Merrill carbine with the original owner's name (a Union cavalry
man) inscribed on the brass patchbox. He was captured, held in Cahaba or Andersonville, when the war ended he was traveling up the Mississippi River on the Sultana - the boiler blew (the steamboat was way overloaded, but Confederate agent Robert Louden claimed he planted a coal torpedo) resulting in this country's biggest American maritime disaster (the original owner of the carbine was scalded to death). Carved on the stock is "C S" - it looks period, I figure the carbine was captured when he was.
I have another friend who has his Confederate major g-g-grandfather's Colt Navy revolver - inside the holster is stenciled the name and unit of the Union cavalryman it belonged to (he was captured, & hanged, on Nov. 6, 1864 - he was participating in "The Burning" of the Shenandoah Valley, & he deserved it!).
The obvious point is a soldier (esp. a Confederate) would improve his inventory if given the opportunity. And since your sabre is dated 1857, it could have started off as Confederate in 1861. JEB Stuart was wearing his U.S. issued eagle sword belt plate when mortally wounded, I think he kept his prewar sabre, too. I don't know what the odds are, but you can always be thinking "maybe" ...