RainSoft ® Hydrefiner P-12 # 9875 - 9879 17561, P12, 17366 13199 Compatible Replacement Water Filter. RainSoft ® Compatible P-12 # 9875 P12, Replacement Water Filter - Fits 9878, 9879 & Other Hydrefiner Systems - # 17561, 17366 , 13199. 6" long by 4 9/16" diameter. - 1/2" UNS Compression Female Threaded - Installation Note: The threaded part in the top of the housing cap is a "nipple". It may come out with the filter or with the housing. Whichever one it is in after the filter is removed the nipple can then be taken from the other item. It is just a small section of threaded tube. 12 per case, sold individually. Order extras and save even more.