This is more or less what we played with. My set is all silver but with different patterns. Some have 4 silver color and 4 brass color. All of ours is in a nicer case though :-)
Ah! They tell them apart by the single and double lines? And shiny...too. For me, that only solidifies it. If they were simply crude cast iron balls, it wouldn’t have the same effect.
But because they are shiny, it makes them seem like a better projectile to roll.