B+W 77mm KSM Circular Polarizing filter for Rollei 40mm f/3.5 Super Angulon PQ lens - new
This fits the Schneider 40mm f/3.5 PQ lens - and I happen to have several used available at the time of this writing.
Kaeseman made the best polarizing material and therefore Schneider bought them and incorporated the technology into their filters. I did extensive testing on different polarizing filters when learning the cross polarization lighting technique for art reproduction and I came to the same conclusion - the B+W Kaeseman are the best! I recently bought several new ones and realized I already had a 77mm so don't need this one. Still in its unopened box.