What are Series 9 filters?
 

I have a Century Series 9 sunshade I bought used, and I have to confess, I don't know what Series 9 is. Is it a size? I see there are Series 9 filters on sale at the Century site. Can I assume they will fit my sunshade? The sunshade accepts round filters, and most of the others that are for sale there are square. I've already sent an e-mail to Century about this, but so far I haven't heard from them.

Series 9 filters don't have threads. They are approximately 90mm, although the glass diameter is less. You use them with different size retaining and stepdown rings. The nice thing about using them is that you can always take them to whatever (almost) new camera or lens you may buy in the future, and all you have to do is buy the proper stepdown ring for about 5 bucks or so--much cheaper than buying all new threaded filters.

Series 9 filters have a 82mm outside diameter and a 78mm clear aperture. My source is a B+W spec guide.

http://www.2filter.com/prices/products/bwregsizes.html

I've never measured, but my lens has 82mm threads and the Series 9 filters are a little bigger, so I just assumed the 9 stands for 90mm. I'll have to check that out next time I happen to have the camera in one hand and a measuring device in the other. (I can tell you that a round can of Altoids is approximately 78mm in diameter, because there happens to be one sitting on the ruler that's on my desk. If the can will fit into the 82mm retainer ring, then that might tell me something.)

Upon further scientific inquiry...your dimensions, Frank, are accurate. The Series 9 appears to be about 82mm, with a glass area diameter of around 78mm. About the same size as the round can of tangerine sour Altoids. Coincidence? You be the judge....