The person who mounted the tangents in the keys mistakenly continued to the next string choir for the first fret, where B should have used the same strings as Bb instead of getting its own choir. When the mistake was discovered the problem with a lacking string choir for the rest of the keys was solved by changing to two triple-frets in the treble. As the keys are close to each other in the treble, with no need for extra fretting spaces between them, this was unproblematic.
To save space in the keyboard layout, the guiding pin and the scribed line on the key is often off the centre of the key. In this way, the distance between the rack slots of adjacent fretting-groups could be closer than the width of the keys.
Tangents have been positioned for a tuning in quarter-comma meantone. This means that tangent positions will be close to the scribed lines for the lowest frets. For a discussion of this choice see Clavichord International Vol.10 No.1 May 2006, pp. 17-20: HansErik Svensson, Comments on the tuning of some fretted clavichords in the Stockholm Music Museum.