The reason the plate changes is that it is a different day and they don't take the same truck everyday.
The license plate of Oscar and Ong's security truck keeps changing throughout the film.
The key for the security box is far too simple in design. There was no need to take an impression and use a rather unrealistic casting process: any strip of metal could have been quickly used to pick such a simple lock. One simply coats the strip with a film of wax, tries to turn it in the lock, and this immediately shows which parts have to be cut away. This technique was already old in the Victorian era, and is not hard to think up for oneself.