Need help installing a Larue MBT

Hey guys, I'm a fairly new firearms owner and am trying to install the MBT into my Colt AR rifle. I've watched some video tutorials and read the instructions that it came with, but I feel like something doesn't make sense. I get to the step where it tells me to unscrew the pistol grip from INSIDE the grip, but when I take the plastic off the bottom there looks to be some sort of foam material at the very end of the inside of the grip. I purchased this rifle from someone who had installed a third-party grip + stock, so I'm wondering if maybe that foam isn't supposed to be there, because I was under the assumption that there would be a some sort of allen wrench screw inside for me to take out. If anyone has any suggestions I would greatly appreciate it. Thanks in advance!