This maybe helpful. . I found out about these locks, two different keys are used for Mills locks, they look identical except one is a snot bit wider.
This basically means that the lock will not accept certain keys, so what I have done is adjust the lock when it is stripped down to fit the Mills key.
I have both model locks, one is a square lock and one is a round lock.the square lock is the one that a larger Mills key won't fit into, see photos.
