Agree!. These lines were available when IM6 rods were made. I have lined all my Loomis IM6 rods with Cortland lines since the day I bought them new. Tried others and they just are not the same...I'm kind of partial to Cortland 444SL lines myself. They were always good on older rods.
I fished with Tom Morgan quite a bit before he got laid up with MS. The reason Winston rods were shorter than their contemporary manufacturers was due to Tom’s experience from his own fishing/guiding that he did throughout his lifetime.I have a 4wt IM6 WT. I called Winston several years ago to find out what line I should use.. They said double taper. I use the cortland 444 DT
I believe the line you liked was the Rio Selective Trout, great line ! I still use the DT on my 3wt Lami glass.When I first bought my 8'-6" 5-weight in the early 90's I lined it with my go-to line at the time: 444SL in a DT5F. It was a very good match. Then about ten years ago I had the opportunity to cast multiple new lines on it and fell in love with the Rio Trout LT (or whatever they called it back then) in a WF5F. I was surprised to discover that it cast better in close and far than the 444SL. I fish the same Rio line in a WF4F on my 8'-6" bIIt 4-weight. They keep changing their name, lines and tapers though because marketing.
Whatever you do, do not buy one of the "modern" WF lines that are anywhere from one-half to a full line size over AFTMA standards for any Winston IM6. They destroy the action and enjoyment of those rods.