I recently bought a Voigtlander Avus 6.5 x 9 cm camera. Would appreciate if anyone can tell me what type of filters (brand, size) will fit the lens. It is a Skopar f/4.5 105mm lens. The thread is on the inside of the tiny lens hood. I have never seen this design before but it must have been common once. Measures 26mm across the front. Are there current production filters available to fit a lens like this, or am I searching for old stock?