As for the red disc, I'm pretty sure that the idea is that you can put your paper on the easel, focus the enlarger (with the red light), remove the red disc, and then expose your print. Supposedly the red light won't fog paper. My enlarger has a similar device, but I don't use it and instead focus on a separate piece of white paper before taking out the photo paper and making my print. I find this much simpler, plus I don't have to worry about any possible fogging. I don't know about the rectangles, perhaps they correspond to paper sizes?