I always thought that the plates were tintype black.They have been shield emulsion, expose and take a negative.The process is bleached with mercuric chloride for imaging. With white on black background of the picture is reversed seem favourable.Hard chrome would be too expensive and difficult both to use as a plate tintype process was that it was cheap and the plates are quick.If polished chrome,so I think it is more likely to be plates.These polished plates of glass used to give glass-like finish on photographic paper.
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