So I dug out a full cup of gravel and silt, along with a small foam ball, a penny, and a nail.

After flushing it with water to make sure it was good and clean, it flows great.

It had a threaded plug to block the cleanout, but it wasn't the right size, and besides the threads in the cast iron are all rusted away. So I filled it with a compression plug. Seems to be working well.

Before: