Dear Mike, Dear all,
talking with leading experts of the taxonomy of Apistogramma and re-reading aquaristic books and publications I become aware that you are right, Mike. The species is not endemic to this lake (which was unlikely, by the way), but known from some places (at least) around the type-locality. It is not even new for the aquarists, because the species is known in the hobby as Apistogramma sp. „Schwarzbrust“ or as A. „Masken“.
I just wonder why Römer does not mentioned it in the description.