Hi...I don't know who the original supplier was, It's a terrible design to work with, I don't think they are making them anymore.... I Have dealt with the same issue as you on both front windows. One way is to push the window out, peel and re'seal it with butyl , then reset the rubber gasket with a tool for setting piping in screen doors. Using a string you would wrap the inside edge of the rubber flange get the window started at the top and then have someone pull the string around the rubber while another pushes from the outside to re- seat the window, Then seal the edge with proflex. My passenger side was done that way. Drivers side I Just spray -foamed the void behind the rubber gasket where it had slid away from the inside wall( about a half inch gap) then sealed the top and sides with proflex and it has worked just fine, of course it's not as good as a complete peel and re-seal, but the foam gave rhe rubber a solid backing on the top front leading edge. I keep a close eye on it and it's been just fine for a few years now. Wish National had just used a conventional window frame with screws, would be a lot easier to repair!
2001 National Tradewinds 7372