Hassy help - What is it?

[DBY]

I just bought a 500cm on ebay - very clean, 1977 build. I am running a few test rolls through it and had a "What?!?!?" moment. Everything seems to work fine - I finished a roll, pushed in the dark slide and removed the back. Now the interesting part - in picture you can see on the back on the right, what looks to be a piece of tape although it isn't sticky. What is it and where could it have come from? I am guessing it might be a seal related to the dark slide "entrance" into the back? The back seems to be built like a brick - doesn't strike me as being user servicable to re-install?