Ezzie: Thanks! Of course--duh--it's got to be poking up not hanging down. I've just had a look and found it: it's a thread from a piece of cloth in the back itself, part of a light seal.
Sandeha: You're far too kind. But it was great fun figuring out how to make the thing. I took my measurements from the remains of the original, which was made of leather. I realise now that with a finer fabric like the blackout cloth I used, the margins between stiffeners should have been smaller. We live and learn. It's got a slight left-hand torque too, but it does fold flat and it doesn't leak. And thanks for the historical info--I knew a little about modern Cooke lenses because a friend of mine was one of the people who rescued THC from the scrapyard several decades ago. Some company had bought Rank Strand and was selling off THC for scrap metal. That would have been a crime.