On page 10, there is a suggestion to enter "STUERBPORAIYSIES!", then enable double-size mode. I can't find any reference for that anywhere. I wonder what it does?

It looks like in double-size mode, every other character is dropped. The manual text hints at the result. "Something strange? Well, just for fun, press CLEAR then type STUERBPORAIYSIES. Now press SHIFT ->. Surprised?"

Oh, I see. The characters get dropped in the transition between normal and double-sized mode.