I am a pianist trying to learn organ and since there’s no funds for lessons I am reading old organ instruction books and came across one from Schneider in the public domain in London published 1875 that has weird signs of “+” for scale fingerings but this plus sign doesn’t mean a sharp or altered note as in figured bass and I can’t understand what it means although I think it is a sign notated in old clavier scores, also.
It’s like my mind has left me with this. Is anyone able to help me out here?