On a couple of trade stimulators, I have the tumblers were removed and a screwdriver or key that fits will operate the lock. Have you tried using a screwdriver to operate the lock? Or can you freely move the cam by hand? Is a key broken off in it? I am wondering if the tumblers have already been filed or removed. Normally, Mills locks will not allow the key to be removed when it's unlocked. It seems odd that your lock shown in the above photo shows it in the unlocked position.
