I'm considering purchasing a used B+W circular polarizer from someone locally. It is described as "B+W 67 CIRCULAR-POL F-PRO" and the seller is unsure of whether it is MC or MRC as he no longer has the original box and it is not written on the filter itself. Here are my questions: 1. What is the difference between MC and MRC? Is there a way to determine which one this filter is based on what's written on the filter: "B+W 67 CIRCULAR-POL F-PRO"? 2. The asking price is CAD$75. Does this seem fair? 3. The filter is the non-thin version. Will I get any vignetting with my Tamron 17-50mm? 4 Anything else I should watch out for other than the obvious things like checking for scratches and such?