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. I can't say I've ever needed to know what that filter was before. As I have now found out, it is a cut off filter that is designed for stopping the red light used. Your best bet would be to contact the manufacturer of the sight, as they may be able to send you another filter, or possibly repair the scope. If you would like, we have many red dot sights available, if you would rather get a new one, http://opticsplanet.com/red-dot-scopes.html
I hope I've been able to help.