Fellow GB owners. I spent hours, days trying to find window felt channel for our 1990 GB 42. Stumbled on these guys at Restoration Specialties
Jeff and his team will take great care of you. The model that fit our aluminum channels is the AS1268. Ours had a plexiglass strip under the old felt, which we needed to keep to get the proper depth seating.
Another item to look for. For some odd reason there were plugs in the drain holes for some of the windows. I took those out and found the channels drained much better. Also, on two of the 12 window frames there was a 1/8" hole drilled into the bottom of the outer channel. This was allowing water to leak into the wall. I plugged those as well and stopped both leaks. We've been in the SE now for 4 months since we replaced the channels, no blow by, no leakage whatsoever in some serious storms. Very happy with the result.