Thanks again, Allegra. That is exactly what I noticed when I re-examined the link you sent. It is very interesting that the extra measure worked out so perfectly for Gounod–I have always been amazed that he could “hear” that beautiful “Ave Maria” melody against the Prelude, and that he made it work! It is a lot like the counterpoint going on in Bach’s “Jesu, Joy of Man’s Desiring,” “Now Thank We All Our God,” and many others. Of course, Bach took the Chorale melodies and really did something different with them!
Incidentally, I do always put in that “extra measure” anytime I play the Prelude by itself, on harp, piano, harpsichord, or organ. I guess I have done it so long WITH the “Ave Maria” that it sounds wrong to me without the extra measure!