This might have been an attempt at deception. Trying to make a lock seem newer than it was.
If you look at the 'R' , it looks to have been over stamped. I would think that, as it was a poor deception, the lock was discarded with its key.

---------- Post added at 10:21 PM ---------- Previous post was at 10:13 PM ----------

This could be a case of deception. If you look at the 'R', it appears to be over stamped also. The seller may have been trying to make the lock appear newer than it was by altering the Monarch. Then, having messed it up , thrown it away with it's key.