I am unsure on the identification of this brachiopod. It is from the Leighton Formation, Maine; which is Silurian, Pridoli. According to Maine's Paleozoic Record (an excellent book that lists all the fauna and flora found in formations across Maine), there are only two Rhynchonellid genera in this formation, Camarotoechia and Rhynchotrema. It does not look like a Camarotoechia, but I cannot find many references for the internal mold of Rhynchotrema brachiopods.
I would appreciate any references you have on this genus, as well as any advice on the identification of it (@Tidgy's Dad