I too, came across Bob at Sidmouth. May have been 78 or 79. That psaltery made the most astonishing sound. I asked him about how it was made (because I was making bowed psalteries at the time)and I recall him saying it had a metal inner frame. A timber frame could not have withstood the huge tension from the strings.
Somewhere I have the photo I took of the psaltery. I bought the LP, of course!