Can't figure out this piece from Hurd double hung windows

Hi there,

I have 1994 Hurd double hung windows in my home. They're aluminum clad wood windows. I've been weatherstripping them all but found this little plastic white cover that's screwed into the bottom, on the aluminum clad (exterior) side. There are two on each sash (top corners of the upper sash and bottom corners of lower sash). When removed I see wood. They also seem to help hold in the jam felt weatherstripping. Many are broken so I need replacements but I can't figure out what these are called, or their purpose. I would just seal them shut with something like epoxy but have a feeling there's a reason they're removable. Any idea if Swisco has a part like this? Thanks for the help!

Other details about the piece:
- No markings except an "R" or "L" to show left or right
- it's about 1.75 inches in length
- the screw fastener is 1/4 inch

Edit: The screw fastener is actually 1/2 inch.