What does that mean?
If you look at the slide, it says "AT5 LOCK" and then a serial number.
Now, on the barrel, it says "LOCK BARREL" with a button behind it, so the "AT5" may have to do with some type of lock or safety on the back of the gun, but since we have seen no other designation, "AT5" seems like a possibility for the model number.
Or maybe I am misunderstanding it and the pistol is a "LOCK" pistol. Hard to say if there was any intent placed on those letters.