I have an idea,
Presume that these locks are commonly used on contractor sites. The contractors have a set of keys that open every lock in the vecinity for ease of use, but once the building is built it would be ideal if there keys no longer worked to prevent security breaches.
Essentialy the locks are masterkeyed, and there is the contractor key, and the resident key. There are master keying spacers inside the lock are the same height as the depth of the blind holes in the plug. When the contractors use ther keys it raises the pins so that the masterkeying spacers are not lifted over the sheer line and stay within the plug. When the resident key is inserted it lifts the masterkeying spacers above the sheerline, and allows it to turn untill the spacers meet the blind holes, at which point the spacers fire into the holes since they are thinner than the top pins. This means that the masterkeying spacers are now stuck in the blind holes and will not come out. The contractor key no-longer lifts the pins to the right height with the master spacers removed, but allows the resident a functioning key safe in the knowledge there is no master key that will open their door.
I am not suer but it's an idea that came to mind.
