From looking at the picture of the crack and frame finish I would say that they are made from Ductile cast iron or nodular cast iron. Not cast steel. Casting steel is expensive. I would bet most of the replicas are made from ductile cast iron.
If brass is strong enough then Ductile cast iron would much stronger then that. Ductile cast can be as strong as a lot of steels. Also malleable.
Cast iron has improved strength to weight ratio over steel.
Better surface definition and finish.
Also I wonder if it was even investment cast for if it was investment cast the finish should be much smoother then it is on this frame in your pictures. shouldn't even need any finish work.
Or at least could be. Finish could be shiny smooth with that process.
If brass is strong enough then Ductile cast iron would much stronger then that. Ductile cast can be as strong as a lot of steels. Also malleable.
Cast iron has improved strength to weight ratio over steel.
Better surface definition and finish.
Also I wonder if it was even investment cast for if it was investment cast the finish should be much smoother then it is on this frame in your pictures. shouldn't even need any finish work.
Or at least could be. Finish could be shiny smooth with that process.