It seems to be a thin celluloid foil, not different from the slides we used to use on retro-projectors. It does indeed kill undesirable harmonics. However, as the old slides, it is very thin, and consequently very fragile. A sturdier material would produce the same effect and be more reliable.
Price is OK (3.55 EUR), maybe a bit high for the cheap material it is made of. It could be even a bit more economic (2.70 EUR, say).