Is there some debate going on as to the "appropriateness" of a stainless steel OEM hubcap from MB versus a chrome plated steel hubcap?
Throughout any car's production run, parts are changed. Vendors for those parts are changed, too, along with some parts being second sourced. Just as one rudimentary example, the paint supplier (G vs H). And, the supply chain of spare parts (which is an entirely different supply chain, corporate wise, from that used in manufacture) used for parts and service operations changes too.
I had dented, scratched and rusty chrome plated steel wheel covers for my 1969. After all, my car was very rough when I got it, so you'd expect the wheel covers to match and they did not disappoint. Some better than others, but all pretty "driver" quality. When I bought a new set as an official MB part, it was stainless. They were supplied by Mercedes-Benz through their parts operation. They are OEM. Is this a problem? Attempting to refinish the originals would bring to mind the phrase "A fool is easily separated from his money".
Though not wheel covers, similarly I recount two other OEM parts I bought from Mercedes. One of them was a check valve/vacuum line for the brake booster, and the other an automatic transmission filler tube. In the case of the former, the original as delivered on the car, looked like it was made in Guenther's garage with scraps including black cloth covered rubber hose, a few fittings, a check valve, and some hose clamps. The OEM replacement from MB was an integrally manufactured unit, neat clean and "all in one". For the filler tube, when I bought my OEM replacement, cadmium plating had fallen out of favor, and in fact, so had zinc; it was painted black. But, it was still OEM and still supplied by MB.
I'd offer that it really doesn't matter whether you have chrome plated steel or stainless; if they are OEM they are the right wheel covers.