I found this post -
"PC - Here's the technical 411 (from my materials handbook): 1045 steel is a medium carbon cold or hot drawn utility steel. It has 43-50% carbon; can be hardened, tensile = 90-110 ksi; yield = 55-85 ksi. Not easily welded. Tougher to machine than hot rolled. Pretty strong stuff.
Cast iron, by comparison, has 1-7% carbon, a tensile of 60-70 ksi, and a yield of 40-50 ksi., is not hardenable, is weldable and easily machined.
Cast iron's main claim to fame for spindles is that it can be cast into the wierd shapes that spindles usually are and, if cast with thick enough sections, is plenty strong enough for the application, but will be heavier than 1045 to handle the same loads.
While 1045 steel is stronger by weight, cast iron is very tough stuff, absorbing shock reasonably well.
The bottom line is if I was rock crawling, I might want the 1045. If I'm just doing hard driving (including some rutted roads at speed) the cast iron is swell. 1045 is not a casting material, so the spindle has to be machined from a large blank of material. The cast iron is economical to cast at a near-net size.
I'll bet there's not a lot of similarity in the prices for these two parts.
There you go."
If Iron isn't hardenable, can it be "forged?" AFAIK (which isn't much), forging is for steel or aluminum. I doubt the Pinto got expensive forged components, or that the Mustang II did either. So if the RMS spindles are forged, and are steel (or aluminum, I doubt aluminum), than they are an in-house spindle; or an aftermarket spindle they got from somebody else. The latter makes more sense if RMS is cutting off the steering arm and welding it back in a different position.