Genuine factory carbon filter for the Hydrotech HT, HTF, 101, 102, and 103 reverse osmosis drinking water systems. Can also be referenced as part # S-FS-19 and also replaces 41400075. Excellent at reducing bad tastes, odors, and chlorine. The 4 stage units require this filter in the 2nd and 4th stages of the system. The 3 stage units with TFC membranes require this replacement to be used in the pre and post filtration positions. Excellent value!