I had the same problem. I place a trap near the coop in the only area I thought something could get into the coop. I caught a very large rat. That was three weeks ago and I haven't had a problem since.
Good Luck
From the time period in 1977 it would not be unusual for a police officer to mark the weapon in that fashion indicating it is part of a crime of some sort. He could then testify in court as to the association of this weapon to the crime by his markings if need be...
A few years ago I had the same concern so I waited for the large safes to go on sale and bought one. If they can get it out of my basement then more power to them as I know waht it took to get it down there.