Cyd,
Some of us have had VHS tapes of Behind That Curtain but, luckily, it is now avaiable in the third Charlie Chan Boxed Set - Side B of Disc 3!
You are correct, sadly, that the others are still lost.
Enjoy the DVD's!!
We have begun watching the Chans in order again . . . we will be watching The Black Camel on Monday night at Rush's Chat Room here so please join us when you can!