Need help identifying button or plug

Hello,

I've ordered the following parts for my non-tilt windows and need help identifying the button or plug piece that plugs the holes towards the bottom of the window.

S800-21 : 21" Series 800 Channel Balance
18-036 : 1/2" Top Sash Guide
S800-17 : 17" Series 800 Channel Balance

The first picture is a view of the back of the button -- it's split in the middle. The second picture is a view of the button installed in the window.

The button is less than half an inch in diameter.

Please let me know if you have any questions which would help identify this item. Thank you for your help with this!