Although they are said to be made in Germany, they are sized more like Japanese nibs - one size smaller than western nibs. This means that a Knox medium nib will be about equivalent to a western fine, and so on down the line.
They are actually quite nice, and they also have untipped stubs in 1.1 mm and 1.9 mm.
If you are worried about your customer wanting only a Bock nib, you could always offer to upgrade when the others are available, if he is not happy with the Knox. They get good reviews on The Fountain Pen Network (which is offline for maintenance as I type this).