The Humble Cudgel is a unique version of the lead pipe. It is less corroded, and has a T fitting on the end similar to a mallet. Its middle section is also slightly bent. The Humble Cudgel deals an extra 4 damage over its non-unique counterpart, and uses 3 less AP per attack when wielded in V.A.T.S. It also has 50% more item HP than the standard lead pipe. However, it has a somewhat shorter reach than a regular lead pipe; most likely a result of the kink in the main section of the pipe itself.
The north sewers are accessed via a door in The Thorn. To get there, enter The Thorn, then follow the north wall, and finally follow the east wall to the corner. Once inside the northern sewers, head down to the bottom center of the map where you'll find a large pipe which is the entrance to the central sewers. Head straight and take the left at the T intersection (ghoul roamers and giant rats populate the area) then look north east and see a red light in the next tunnel. That light marks a door, inside will be the corpse of "Blind" Luke. Take the key off his corpse, as you'll be needing it to access the sealed door. Next, head southeast through the map to find a sandbag wall and a couple of NCR troopers in a tunnel. Go West past this tunnel and up a couple of ramps to find a group of about five Fiends guarding a door. Go South through the door and the next room to see a set of metal stairs. At the top of the stairs is the door you needed to get the key for. It leads to the sealed sewers. Inside the sealed sewers there are a handful of assorted ghouls, including roamers, reavers, and glowing ones. Kill them and in the first room on the left will be a dead prospector, loot him for The Humble Cudgel.
Alternatively, one could just access the sewer grate outside the entrance to Camp McCarran and continue from where the NCR are at the sandbags. This must be done after the key is acquired, either off the corpse of "Blind" Luke or that of "Sweet" Jill.