I seem to recall reading that as a kid too, but I can't find it now. I keep finding references to "Encyclopedia Brown, Boy Detective" about a Civil War sword being fake (instead of a Great War one), but with the same plot I'd remembered.

The Encyclopedia Brown story I remember reading as a kid involved a Civil War era sword with an inscription saying it was given on the occasion of the First Battle of Bull Run. The clues that the sword was a modern fake were the phrasing "First Battle of Bull Run", but also that the sword was gifted on the Confederate side, and the Confederates would've called the battle "Manassas Junction".

The wikipedia article https://en.wikipedia.org/wiki/First_Battle_of_Bull_Run says the Confederate name was "First Manassas" (I might be misremembering exactly what this book I read as a child said). Also I'm pretty sure it was specifically "Encyclopedia Brown Solves Them All" that this mystery appeared in. If someone has a copy of the book or cares to dig it up, they could confirm my memory.

Can confirm, it was an Encyclopedia Brown book and it was World War One vs the Great War that gave away the sword as a counterfeit!