I'm missing "Insulation Plugs" on Sliding Glass Doors

I have a number of sliders all of which are missing "insulation plugs" at the middle of the upper track. There are no identifying markings anywhere on the doors/glass. The installs probably date from 2003. To help ID the parts, I posted three pics: a view of the outside of the door, a pic of the spot where the missing insulation plug attaches to the track, and a picture of the lockset, which I removed to see whether I could find a manufacturer's stamp (I couldn't).