I came across an older book on productid brachiopods by famous British paleontologist, Helen Muir-Wood. Link: H. Muir-Wood and G. A. Cooper. 1960. Morphology, classification and life habits of the Productoidea (Brachiopoda). Geological Society of America Memoir 81.
The book was apparently in the library of German-American paleontologist, Curt Teichert, who edited many of the Treatises of Paleontology. Link.
Some of the brachiopods were, to coin a phrase, exhibiting derived convergent evolution with the mollusk rudists. Both were: elongated with one large and one small v