Some of you may be asking yourself "Doesn't Strophostyles helvula have lobate lateral leaflets?". That is where I was a bit tricky. This is actually S. helvula var. missouriensis which lacks the lobes on the lateral leaflets (the typical variety has lobed leaflets).
Vegetatively the genus Strophostyles can be distinguish from Amphicarpaea bracteata, which it most resembles, by its erect stipules. Amphicarpaea looks exactly like Strophostyles helvula var. missouriensis but has appressed stipules. By clicking on the photo above you can see the stipules in the enlarged version. They are perpendicular to the stem.
Lesson learned; Librarians make great botanist. If only it worked both ways.
Thanks to everyone for playing!