Thanks, Vestia. I'm fairly certain I have the Stigmaphyllon. I think it is in flower now (and definitely has a lot of the brown "butterfly" seeds), so I should be able to get a photo.
But are the two species actually different plants?? I can't tell any differences in the photos in Plant Files, but it may be that some of the photos have been incorrectly attributed to one or the other (or both!) of the plants.
Jeremy, I have two different yellow butterfly vines - both sold, from a nursery, as Stigmaphyllon. They are obviously different cultivars - one has LARGE yellow blooms and the other has small, quarter-size blooms. The seed pods are way bigger on one than on the other.
I just looked both of these up to verify and Stigmaphyllon littorale seems to have a name change to: Stigmaphyllon bonariense
They do both look the same but are accepted separate names. I've sent in an error report for correction.