I think if you squint your eyes enough you will see that the reflection in the glass just makes the barrel look round.
It looks like an octagon barrel to me.
The brassy looking light off of the cylinder is also due to a light reflection.
As for all of that brass, the only brass parts I see on it are the trigger guard and the grip strap.
Almost all of the American made Colt 1851's had these brass parts but they were silver plated, like the one Claude shows in his post.
There are some American made 1851's that have steel grip straps and trigger guards and almost all of the London made 1851's had steel grips and trigger guards. (The British didn't like brass).
The bare brass on the gun in the photo is very likely due to the silver plating being polished off because it was tarnished. It's very common for this to happen.
So common that I suspect the gun in Claudes post has been replated.
Just for giggles (and to make them look more authentic) I silver plated all of my Italian Colts. The 1849, 1851 and 1862 are shown.