When is the contents of a Lockbox determined? At the time it is found in the clockworks? or at the time it is opened?
Can old lockboxes from before the Shadow Key update contain Shadow Keys if they are opened currently (after the update)?
The question is really for the developers, but if anybody has a Shadow Key that they are sure came from an old box that would answer the question too.
The answer is yes so sayith Boswick:
http://forums.spiralknights.com/en/node/30115#comment-179181