This is an old NOS #85 Series V, 30 mm diameter, drop in type color filter. Made by Endnalite many moons ago [back when such things were still manufactured here in the United States...], it is fully coated and in new condition. The #85 is a warming filter (minus blue) good for use on the moon, Mars, Jupiter (for bluish festoons), and for enhancing cometary dust tails.
Some scopes, including some of the smaller Maks, use this size filter. I have also modified several of my 1.25" Barlow lenses to use these Series V drop in filters, and it's actually very convenient not to have to screw them into each eyepiece. Asking $6.50 including postage w/in the USA.