To separate female oblongana from gentianeana, Knud Larsen writes: The genitalia of gentianaeana is one third bigger than oblongana mainly caused by the length of the second part of ductus bursa (after the swelling of the cingulum). This part is also often broader. Signum is larger like a big bowler hat turned upside down.
In gentianeana the medium part of ductus bursa - the swollen part (cingulum) - often have small sclerotized corns seen in large magnification.