I've looked for this part for some time. The part is really old and brittle. I have included a photo and drawing.
The screw that holds it in place also holds the frame together so, the placement of the screw hole is important. If I could find something close I could drill a hole to match. I know that would void any warranty.
The part restrains the channel balance. I have four different size channel balances (block and tackle). all of the top sash guides are the same size regardless of the length of the channel balance.