Here are some plastic reproductions of the thing that sits in the Black frame at the back (I'm sure someone can tell me their proper name):



Here it is slotted into position. The printed one is on the left, an original on the right:

Hi Dan
I have looked in my parts list and cannot find that part even listed.
Rear frame insert is as good as anything, someone may have a comprehensive list and put the correct name to it soon.
Mick