Sorry couldn't resist.
The date and geography in which you're interested will impact the following generic reply:
It depends on what "fusil" you're talking about. A French fusil as with their muskets, steel. An English officer's fusil, brass. An English trade gun often called a fusil or "few-zee", again brass.
Give more details and you will get a much more precise answer. Sorry I had to be generic.
Looking at their site, and what they are offering. That style guard was always iron, never brass on originals. If you want authentic choose walnut wood and iron furniture for this style kit.