This location is part of Salem and has an armor workbench on the second floor, ammo boxes on the attic, and the basement trapdoor that can only be opened using a key provided by Barney Rook.
There should be an NPC named Barney Rook besieged by mirelurks. You should be able to get the key to the basement from him after helping him with a quest. However, there will be an unfortunate glitch where you'll find two dead mirelurks in front of his house and he's nowhere to be found. This will prevent you from getting inside the basement where you should find a copy of Guns and Bullets inside.