This is a used, but not abused, Meade #905 Variable Polarizing System. The # 905 Variable Polarizer system is 2 Polarizer filters mounted in a specially-machined cell, for glare-reduction in observing the Moon. Rotate the thumbscrew at the side of the unit to achieve light transmission between 5% and 25% of its original value. The #905 inserts into the diagonal or focuser of the telescope, followed by an 1.25" eyepiece.
The advantage of this design over other polarizers, is to use the other polarizers one has to constantly take out the eyepiece to readjust the amount of polarization. This is exclusively for viewing, for photography you would use individual polarizing filters.
Asking $35.00 plus $4.00 for USPS Priority Mail shipping. Payment by PayPal or money order please. Will ship to US address or Canada (Canadian shipment add $2.00).