The "Holga Flap" is a bit of gaffer tape that covers the frame index hole. It probably isn't strictly necessary, but light leaks suck. There is a 14 ? mm disk of red(ish) translucent material INSIDE the camera body, recessed in the wall behind the film plane. I 3D print mine from translucent red PLA, but you can use a plastic file folder from an office supply store (like $0.99). Make sure you can read a dollar bill through it.