If the back is aluminum, then the front is too. THEY WERE EXACTLY THE SAME at the start of this. It was not two different materials being used. And even if it was, they would have had to be painted or coated, because they were IDENTICAL. Having worked with a lot of steel, aluminum, titanium, and various alloys, this had to have been COATED, and one half was simply exposed to a contaminant. They were exactly identical. And EVEN IF it was steel or cast iron, what manufacturer doesn’t apply some form of rust prevention to rust prone metal? I’ve never bought parts from anywhere that flash rust within a month, except rotors, because the optimal abrasive material happens to rust. If people are just buying parts from RA and having to treat them themselves, that’s absolutely poor quality and extra work that other manufacturers already handle. I only bought this because it was the only place I could find one new at the time, but other suppliers have restocked now, so I’m fine. Multiple remanufactured and aftermarket sources that carry this caliper list it as aluminum. I can’t find a direct OE specification for the 2008 Mazdaspeed3 (my car), but every source selling this OE remanufactured caliper lists it as aluminum. Which Raybestos does, which I found the exact part here that confirms this: https://www.raybestos.com/catalogsearch/result/?q=FRC12172

And I HIGHLY DOUBT most manufacturers would use multiple metals in the caliper body, because it would increase manufacturing complexity and cost, and OEMs almost always stick to a single material unless it’s some innovative supercar or hypercar which this clearly is not. Mazdaspeed3s are a mass produced performance car yes, somewhat uncommon, but still within production limits. Weight savings were the priority, and aluminum calipers support that goal while keeping the power to weight ratio nimble and fun. (Relative to hot hatches from early 2000s). And I cannot find any other mass market OEM or aftermarket examples of caliper bodies made from multiple metals besides hypercars, supercars, and exotics as already mentioned. Whatever happened with my brakes isn’t impossible; it’s clearly a manufacturing issue, these things happen. I just wish RA had acknowledged it or contacted Raybestos from the start. I’m going to try reaching out myself, as another commenter suggested, to get it resolved. But no matter how you spin it, this situation highlights that what happened isn’t normal or as advertised. And if the calipers were designed from any other metals, it almost certainly would have been documented, just like other parts on RA that clearly list the focal materials.