Brass Battery Caps with Black Chrome Finish; Stripped with Yellow Contamination

Hi, I am a customer rep at a coatings company in Hudson, Mass. We have a brass battery cap customer that sent us a few hundred pieces that were stripped by an outside vendor. When we received them from the customer(stripped), they all have a yellow residue that is coming off on our gloves. The customer wants us to "clean" the parts, but because we don't know what the contamination is, we run the risk of contaminating our facility. The vendor said that they stripped the battery caps down to the metal, if that's true, why the residue? Any ideas? The customer says they were virgin parts finished with a black chromate and then stripped.