What is this tiny plastic insert in my sliding window sash?

Question

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!

ojait · Accepted Answer

By looking at the space between the window frame and the sash (approximately 1/4 inch ?) I believe those shims are installed to keep the sash straight and from rattling around when it is opened and closed.
Or they could be part of the spring assisted lift system of the window. The piece that is in view may be attached to a mechanism on the edge of the window which compromises the spring lift mechanism.
I'd say some sort of shim, though.
It may be easier to find the manufacturer of your window and get the shims from their site.

What is this tiny plastic insert in my sliding window sash?

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