If the rear seal set has the pipe cleaners instead of the fiber side seals, then.. the reason is that the instructions would/should call out to fill the cavities with RTV (a small tube of RTV should also be included in the set with a long/narrow pointed end), and pre-soak the pipe cleaners with water, then insert the pipe cleaners into each cavity and let it remain there as installed... that will allow the RTV to cure by having moisture present. When I worked for FP years ago, that was an alternative sealing choice we had.. so.. it may now be the current situation as packaged. (Also, I believe the fiber side strips were being sourced and they were relatively costly, possibly eventually discontinued by the supplier.)