I would like to present to you not very known keys: the " Cylinder Lockblocking Keys" for very different types of locks such as Mortice-, Warded- and Furniture-Locks.
This are involves "mini-cylinders" which we introduce into the bigger locks, which stay inside to prevent the lock-picking.
I possess only 4 of this keys:
- one FLORA
- one BURG WÄCHTER
- two SCHLOSS KÖNIG
The pics :







The little part in brass, thus the small cylinder, has between 6 and 7 mm in diameter, it stays in the lock by rotation of a spur (for the FLORA and the BURG WÄCHTER).
For the two SCHLOSS KÖNIG, it is different, it is a system by screw, with threading to the left, and there are two fins of steel which maintain the cylinder in place in the lock.
We find them in hardware stores in Europe...
Sorry for my bad English, but i am French.