A very common brachiopod genera here in the US is Leptaena and it appears to also be common in the Skaly beds of Poland. Below are specimens of Leptaena analogaeformis with both internal and external views available.
Compare that to the interior of the Brachial valve - Note this is a separate specimen and was not found articulated with the previous specimen.
Unfortunately I don't have any specimens with both valves articulated. Both specimens came from near Kielce, Poland. The Skaly beds are dated to the Middle Devonian ( Eifelian to Givetian stages).