Window Sash Channel Top

Your help please in identifying the correct replacement part for the plastic piece at the top of the window sash channel. This piece is screwed into the window sash itself and keeps the channel balance with the sash.
The top is flat and a 7/8" square. On the side view, the long side is 1.5" and the short side to the indent is 1/2"; the indent is 3/16"; then another 1/2" to the slanted part. The slant is 5/8".
The part into the channel is 5/8" wide in both dimensions.
The top is flush to the side with the indent/slant, but extends out the other three sides.
You have been fantastic in helping me repair my windows and getting the correct channel balances to replace, but I now have some windows with this piece broken out and the channel balances will not work without it. Thank you for you help once again !!!