Having revisited the eBay site and had a look at the clamps in detail, i need to correct my earlier post.
it looks to me that the clamp is stainless steel, but the hardware is not.
The bolt head has 10.9 stamped on it which is usually the tensile strength. Which I believe is stronger than stainless steel can achieve so my conclusion is a steel alloy, which will likely corrode just like the originals.
They’re probably the same clamps that BMW use, but a lot cheaper……