Hello!
I couldn't find that plastic end cap you show in your photos. It appears that your shoe stud glide looks similar to the 36-078 which takes the track 36-078A, but as far as I know, these type of tracks do not use that end cap you show in your photo. If you want, you're welcome to send us photos of the full operator assembly along with dimensions.
I'm afraid we don't sell this operator. If all you need is that cap, I wonder if it'd be possible to take some type of tube or cap like object and use that for your missing part.
If you do want to replace the track on the window, take a look at the 36-078A which takes the 36-078 shoe that looks similar to the one on your operator. Also, take a look at the newer style Encore operator, the 39-239B. It uses the 39-536 pivot slide which works with the 39-556 track. Take note you will need to retrofit your bottom sash stop to accommodate the wider operator body of the 39-239B. We can't promise these parts will work for you since they aren't direct matches but take a look at the specifications and see what you think.
