This is a frontal flap on the treble side of the lid. If the instrument was played with the lid down, which was normal during the period, raising this flap would increase the volume a little. Slamming the flap closed could imitate gunfire in “battle” pieces.
A view of the flap lifting mechanism, a vertical pusher covered with leather.
A view of the tuning pins and bridge. The pins are arranged in two double rows, with adjacent notes alternating right and left. This made it challenging to find the correct pin while tuning. Some form of counting pins was necessary when setting the temperament, as described in Claude Montal's Art of Tuning.
Note that the bass string windings are somewhat open, and extend across the bridge. The bridge is undercut at the bass end, to allow the small soundboard to vibrate more freely.