That rectangle is actually the back side of the grip section. It sits against another face in the shoulder rest section. It is not a support as there are no supports in this file. Each section has been designed to be built as its own stand-alone pep project, and joined together in the end to form the rifle.
Edit: Woah, I was wrong! That rectangle is actually a pointless rectangle that made its way from the model to the pep! That rectangle can be ignored. It has no bearing on the rifle itself nor does it even connect to anything, so take note of that. Dang, one error!
As far as printing in black and white, I don't think you will be able to tell the difference between red and blue, you may have to changed it back to dots and dashes. The reason I use the solid lines is because sometimes with the really small tabs, you can't tell if it is a dotted or a dashed line.