According to the "seriously fish" website, if the black stripe extends from the lower lobe of the caudal fin as far as the gill cover then it is T. boehlkei.
Indeed, “hockey-stick tetra" is an alternative (although rarely-used) name; I’ve always known both Thayeria boehlkei and Thayeria obliqua.as penguin tetras (or penguin fish). There is also a third species Thayeria ifati which I don’t recall ever seeing.
