plastic internal frame

Thanks for posting. It appears your are referring to the window glazing bead. I am unable to identify which one you will need based on the balance series. 

I'll need to see a photo of a piece of the glazing bead, from an end profile view, to recommend the best replacement. Let me know if you have any questions.

Thanks for your photos. We don't have an exact match but our 55-062-6, 55-193, and 55-206 are great places to start. You'll need to compare the dimensions shown on the diagrams to confirm which of these may work best for you.

Here is an example of the dimensions for part 55-062-6, which can be found on each of the part links above. Let me know if you fell they will not work and we can try to look for other solutions.

Profile view of 55-062-6