Your experience is quite interesting, LDH. The conventional wisdom of using a lens shade is to prevent stray, non-image forming light from striking the front lens element, and thus bouncing around between lens elements and the film, reducing contrast. I wouldn't have expected the same benefit on a pinhole camera, due to the lack of a lens. But, I suppose there is a possibility of non-image forming light reflecting off the inside edge of the tiny aperture plate, causing contrast-robbing flare.