If the post is about locks, safes, or access systems: An "easy mark" might refer to a poorly secured lock or system, but adding a "serial number" suggests traceability. For instance, a lock with an exposed serial number could be used to look up a master key code online — making an easy mark even easier to exploit.
Open Easy Mark Plus on your Windows computer. A prompt will automatically appear if the software detects it is currently unregistered. Step 2: Enter Registration Data easy mark plus serial number