[Marcel van Heijkop]

Hi,

Judging from the picture on page 41 of Mick J. Prodger's "Luftwaffe vs. RAF - Flying clothing of the air war 1939-45" (Schiffer, ISBN 0-7643-0234-5) it could indeed be a red filter of a pair of Night Vision/Anti-Searchlight Goggles.
To be precise: Your piece of plastic looks to be the corner of the filter where it is attached with rivets (can't see how many) to the lever to pull the red fliters over the ordinary lenses.

Quote from the book: "It had already been discovered that the use of red filters could enhance or at least accelerate the ability of the human eye to adjust to night vision. It was also known that, once adapted to night vision, exposure of the retina to white light caused immediate night blindness. Special goggles were therefore developed for night fighter pilots by the Nitsche and Günther optical company in conjunction with Dr. Knöthe of the Luftwaffe medical research facility. The goggles comprised clear glass Nitsche and Günther lenses in a rubber padded frame which fit closely to the face, with red filters that could be pulled down in anticipation of enemy activity and bright lights, thus preserving the pilot's vision."

Hope this helps

Best regards,

Marcel