After killing Sarge, you'll enter another level where you need to find the PDA of P. Rogers to continue. Right before you find this scientist, there will be a room filled with 4 stone tablets that are being researched. If you notice the one on the far left, it almost exactly resembles the original box art/title screen for the original DOOM. The only exception is that the lead character is holding a cube, and the top/right corner of the tablet is cut off to where you can't see his face.