Milgard window sash replacement

A home owner from California says:
I'm unable to find the part number to order this item. I ordered the corresponding channel balance (27" Series 380 Channel Balance S380-27 option: 2640)and it worked perfectly. However the plastic stopper on one window is broken.

Please let me know the item number so I can order replacements.

Thank you.

User submitted a photo of a top sash guide.
Profile picture of Paul
Paul from SWISCO responded:
Hello, Linda. I recommend looking at our 18-188. This looks like it could be a good match but see what you think.
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