Our thanks go out, as always to those generous UK suppliers who provide us with luggage. I just received your No. 3 Issue. I found the Feedback on Jim Brockman's concave style interesting. I make some comments in my piece. The ridge concave spine is best suited to thinner or normal calliper paper depending on the size of the book-block.
Stiff paper needs a different treatment, as does wrong direction of grain.
Stiffer papers would not open and lay flat readily near the ends of a concave spine with a rigid lining.