Just a follow-up. With the help of the endoscope, a wire coat hanger, and an overabundance of determination and patience, the 15mm is back home with it's brethren. Fortunately, the well where it was trapped was too small for it to fall down flat, and the box end was up, so I was able to hook it and fish it out.
What a PITA from a simple fumble.
What a PITA from a simple fumble.