Thanks for the re-assurance. My diesel mechanic did not seem too concerned about the small part, likely residing in the oil sump, though he stated it was not a "good" thing. So, I did not panic when I discovered the missing part.
I am going to completely clean that engine and associated bilge, under the engine, today and tomorrow. That should afford me the opportunity to glue a magnet to bottom of the sump. I will verify that it is aluminum first, or that idea will be of little value.
Have a great day.