Seven locks

Apparently special "master keys" will open these locks, and these contain tubes containing the keys to this block. So just keep the most recent keys in the tube, then the posties or sparkies or whoever can be given the master keys and still get in. But as you can see, some of these have been damaged by "anteaters" trying to tunnel in...

 

More: www.flickr.com/photos/arabella/244775814/in/pool-whereinhel/

Done