Lumicon Oxygen-III filter, "for very faint nebulae." Lumicon sold the filter as "cosmetic", and I purchased it from them at the Riverside Conference. The filter exhibits a mottled appearance near the very edge of the glass, which is visible in the photograph. Despite this, the filter works very well, and I have used it on many different nebulas. For example, the O-III with my Tele Vue Pronto will easily reveal the Veil and North American nebulas suspended against a pitch-black sky.
$65, check or MO