they have the prefilter that fits just about all cone filters. I have the RAISS with the S+B filter and the correct size is 20-2174. Avail in different colors too. If you call them at their 800 help number they will know EXACTLY which one you need. (be sure and have the measurements of your filter if its not the S+B.
the outerwears unit is great. 10000x bettter than a panty ho. they're cheap, and they do their job. I had one on my S&B for probably 2yrs and only cleaned the pre filter once a year. S&B still looked like new when I sold it.