The (as yet unpublished) Thomas Watts MS from the Peak District includes dance music, church music and one or two classical pieces. A few are captioned 'for the German flute' (which might suggest that the remainder were written out for a different instrument?)

In every source I've seen (and that means a lot) "for the German flute" means the tune has been transposed up a fifth from a version primarily intended for the fiddle, where it had a range that took it down onto the G string. Usually the result is that the flute version goes above where first position on the fiddle will take you; typically these flute arrangements go to D above the treble staff. Nobody would have played the flute and violin versions together, and usually the flute-specific version was intended for solo performance.