With the recent cold weather, I've been trying to best insulate my house and keep it warm while not using too much energy. While applying weather stripping to my windows, I noticed that some of the windows have these tiny plastic inserts between the sliding window sash and the frame. See photo. I can move them a bit if I poke at them but they appear to move fairly smoothly with the window when opening an closing.
I see them at all four corners of the windows that have them and my BEST GUESS is that they are used to ensure the rubber weather stripping is pressing as tightly to the frame as possible and making a proper seal.
My question is what are these tiny inserts called, where can I get them? I have several of my windows that don't have them (presumably fallen out/gone missing) and I have noticed those windows don't sit as tightly into the frame as the others. I've looked up every variation of "plastic" + "insert" + "sliding" + "window" that I can think of and haven't gotten anywhere. Poked around my local big box store's window section and didn't find anything.
Appreciate any help!