Because the two 8’ choirs on a 3-choir instrument are separated by the 4’ register, the back 8’ stop on the 3-choir instrument plucks farther from the nut and produces a greater tonal contrast to the more nasal tone of the front 8’ choir. I find the back 8’ choir on a 3-choir instrument to be prettier—it’s identical to the tone of the lower-manual on the Flemish double I have here.
You asked about Flemish papers. The harpsichord builders in 17th-century Antwerp (the center of Flemish harpsichord building) belong to -- same guild as the painters and printers. The harpsichord makers could make use of the skills of their brother guildsmen. The harpsichord makers bought printed paper sheets and strips from the book printers (printed on screw presses using carved, inked pearwood blocks. The papers were saturated with glue and attached to the inside surfaces.
The two most famous Flemish harpsichord makers were brothers, Andreas and Iohannes Ruckers. Andreas tended to use this pattern in his keywells:
I used this pattern on the 3-choir single which has a copy of an Andreas Ruckers soundboard painting:
Smaller strips of printed paper were glued around the soundboard:
Iohannes Rjuckers tended to use this pattern:
I used this pattern on my own Flemish double:
On one of the doubles I have here I added a painting copied from a later Iohannes instrument:
The French style of soundboard painting is more “painterly” and realistic. Printed papers are not used.