I am a professional organist who has been self -teaching improvisation from an organ improvisation textbook by Marcel Dupre, titled "Complete Course in Organ Improvisation, volume 1."
At this point, I am half way through the book, but have been using it from beginning with my piano students and find myself unable to explain to them several Roman numeral symbols as I never encountered these ones before. I am referring to the IV with a 3 and IV with a 5 above it in the attached picture. Also, the I and III with a 5 above it.
When I first did these exercises myself, I played the IV and I in root position unless it had a 6 or 6/4 above it. I have asked several professional organists re this and got either no answer or advice to play this in root position. Anyone have any ideas? .