The photo is from a series done by someone at Mathew Brady's Washington photo salon, and he actually has TWO bayonets.
The obvious one is on the end of the rifle he is holding, but there is another bayonet to Brownell's left, in the scabbard.
The image has been cropped to show the bayonets, but sources say that Brownell is certainly wearing his original Fire Zouave uniform.
If you look at his belt, you can see the letters M. I. E. R. These are the last letters of the word "PREMIER," which was the name of the fire company to which he belonged before becoming a soldier.
Now--which bayonet is the "real" bayonet?
Is the gun/bayonet combo only a studio prop?
Is the gun the actual weapon that killed James Jackson, with a prop bayonet, making the side bayonet the real one?
Why would he have a prop bayonet in a real gun?
Why? What? How? Hmmmmm.