This is a scaled down version of the fire hydrant bat that the Behemoths carry. It is it's own standalone weapon. You can find it located in a Vault 111 trunk next to the Vault 111 elevator platform. I've thrown in some other goodies with the trunk as well. 

The weapon has 125 base damage. Fast melee speed, reduced weight, increased value, and only 5 AP cost in VATS. 

Simply download and install with NMM or download and copy Data folder to your Fallout 4 directory.