I'm convinced this is an early instar of Dasylophia anguina. While I haven't seen any specimens as yellow as this one, the black "shoulder" spot, shiny black "button" on A8, raised rear end, elongated anal prolegs and other features scream "black-spotted prominent." Ignore the coloration and just compare physical features on these guys:

And here's the green form, though still not as yellow as yours:

Wagner lists sweet clover as a foodplant. Hope this helps, even with the year-long delay!

It looks a lot like a Hyles lineata, but is too yellow. However, it is in the process of molting or has just molted to a larger skin as evidenced by the old head capsule/shell next to the real head.

Can you identify the plant species? That often helps. Are we looking at seeds forming on a sweetclover?