Though I live in Australia now I was a volunteer at the CRRM for over a decade. During that time tenders were switched all the time for steam ups and service reasons.
I last saw the real 318 Tender behind an RGS engine at Knott's Berry Farm in LA., #42????
You have to remember that these things have been rebuilt so many times it may be hard to keep to prototype. Metal does rust thru. One example was when we brought in the #191 (DSP&P) from the Great Lakes area. It was pretty original (tender) but one night we were in the office after dinner and it was pouring rain. We heard a loud noise and went out to find the tender had fallen in on itself. Though paited nicenly outside it was just rust being held together by paint!
Cheers
THOM