Had a good look this morning after my daughter put a film through it.
It does not touch on anything, I think the light seal suggestion must be the right purpose.
We both saw it under the spool, but it was
upside down !
As soon as I took the spool out, it fell out. (My daughter had handed me the back as she could not see how to release the spool
As it does not come into contact with anything, it won't require much holding in place, so just busy with a scalpel cutting some Tesa to fix it in place, which will actually be stronger than the 1/8" of plastic that held it in the first place.
Then we will put another film through it, and develop the two, firstly that will confirm the camera operates as it should, and it will also show if light seal was its original purpose
I hope the seller will have the decency to offer her something towards another back, assuming the camera is working properly, we will then decide how we respond to his replies (if we get any more).
The fact that we both saw it behind the spool, and both thought it was some kind of spacer or guide, confirms to us that it was like that when we received it.