Master Key for 1600 Series Master Locks starts with a "F" and has 3 numbers after it. The master key will open all locks associated with that master key. If you do not know what master key you have you must destroy a lock and look under the black dial portion of the lock for a red flag that is 1/8"x3/8" with a stamped key number on it. This lock will be destroyed completely and will not be able to be used again. You either are buying locks with master keys and need to order as new series or you have an existing master key series that start with F or S.