La réponse de Schneider Optics :
Hi Jean Claude,
The difference between these filters is in the polarizing foil - SO3 is our regular foil and AUC is Kaesemann.
A description of Kaesemann is pasted below from our website FAQ section:
6. What is a Kaesemann polarizer?
Käsemann was an independent German company renowned for manufacturing some of the highest quality polarizer material in the world. Schneider Krueznach purchased the company in 1989.
The quality and consistency of Käsemann material is uniform and neutral in color. In addition, the latest cementing technique used on these filters is guaranteed to prevent delamination. Moisture from humidity will destroy polarizing material if delamination occurs and this cementing technique protects the material by preventing delamination. Käsemann Polarizers are available in Linear and Circular. Note: The circular type comes with MRC (Multi-Resistant Coating) through 95mm.