These locks have a highly elegant and advanced mechanism. The keys have a distinctive elongated shape with a flat ring and assymetrical wings at the end. But the most striking feature is the ingenious butterfly shaped, padlockable double keyhole cover that gave these doors their names.

The door and was known as a 'butterfly' because of the distinctive look of its padlockable numbered cover. The butterfly cover is polished and the numbers repainted by hand with a one-hair brush. By turning it counterclockwise it closes both keyholes and it can be secured by a padlock. See a picture before restoration. The door itself is made from 0.4'' steel and is litterally bulletproof. It weighs about 4kg and is solidly fixed to the shadow box.