Hello John! I asked Ray, who is one of the founders of Dusty Strings, and he has a memory of a harpist in Seattle having a fleet of rental harps that were made of fiberglass, which could maybe look like a metal soundbox in a photo. He thinks they were made by a guy named Earl Thompson in Maryland, who also used the name Linrud Harps. I'm not sure about the Seattle connection, unless this person bought her harp from someone in Seattle, or perhaps the builder lived in Seattle for a while... Ray thinks the company probably isn't around anymore, since his last memory of them is in the late seventies.
For a string chart, our advice would be to contact Robinson's Harp Shop. If anyone is likely to have specs or information about those harps, it's them! http://robinsonsharpshop.com/