Size/Shape - Usually rectangular, sized thin enough to ensure the pyrite centers in the middle of the pan (fore to aft).
Some grind a cutting angle on the otherwise square edge that bears on the wheel, like a positive rake to a lathe cutting tool, but I’ve never found that necessary, at least as tested on 5 wheellocks, 2 being originals.
Crumblies - Read here, they not only work, they can work really WELL!