I found the message "You get the rest of the credit from handing in the paper homework".
Michael Gage had written in an earlier thread, "The student sees these messages at the bottom of their screen." That's what confused me...
The message in question actually appears as a Note at the bottom of the problem statement -- not the bottom of the screen. Before the buttons for Submit, etc. And quite above the place where the student sees the report at the bottom about attempts, score, etc.
I hope my students will be more eagle-eyed than I in finding it.