I saw a book called Verdi's Middle Period. Personally I'm not an authority on opera, and the proof is that I didn't even know that Verdi had a middle period. Funny name, that:
Guiseppe . Verdi

I guess this is where the "extra" periods go when people (e.g. Harry S Truman) take on middle initials sans periods.

My other theory (not having read the book) is that "middle period" is a synonym for belly button (in operatic circles).