I have been practicing this piece (The Stars and Stripes Forever) for a while. On the first note of the second measure of the second line, there is a natural sign. I have always understood accidentals not to affect notes outside of the current measure. If so, why is there a natural sign on the C while the key is F Major? Is this a mistake, or an obsolete practice (this is from 1897)? If the natural sign were not there, would the C be played as a C#?