C7ZZ-222A55-A Door window rear stop, which installs to the guide (C7ZZ-65222A57). Stop adjusts the door glass in its up most position. One per door, fits left side. This piece falls down inside the door when the rear guide is removed. Bolt included, but may not be exactly as pictured.
You can find the reproduction here.
Check out our handy diagram below to find other '67-'68 door parts you may need. All the item numbers shown can be typed in to the search bar on our site. (Click image to view full size) 
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WCCC is a buyer of used and NOS hard to find 67-73 Mercury Cougar parts. Usually we give an in store credit of 50% of our web price for parts delivered in hand or by carrier to our Brooks, Oregon location. If you're not interested in store credit, just shoot us an email and we will let you know if it's is something we can pay cash for. We do not send cash for parts in advance but we often can compensate you within hours of receiving the goods.
Sometimes a 50% wholesale bid will not be enough to entice you to sell. It never hurts to ask if we can up our bid! If it is an in demand item that we have requests for we often can pay more. Just email us if you have questions or want to share pictures.