That is strange indeed. P. filamentosus is not a Sumatranian form but one from Kalimantan. This should be a mistake of the importer, not the exporter.
Yes, it is possible to differentiate the females of gunawani and filamentosus but for certain in courtship behaviour only. If not in courtship, it is the distinctly stouter body structure only that indicates gunawani. In courtship, the female filamentosus assume prominent colours in the unpaired dorsal and anal, namely rather broad brown bands that are very distinctive.
But I think this will not help you very much in the normal everyday appearance. There, these bands are not to be seen at all. In this case its the stout body form of gunawani only that would separate both.