Hi John, nice set of cars. To answer to some of your questions I will say at first that there is many variations around those cars. They may be founded with different type of couplers, die cast and brass but I have never seen spring loaded buffers, they are certainly non original, we must be careful with pieces coming from great Britain, many of them have been altered by modelers. Couplers may be drop link or automatic for the latest ones. Here some examples from my collection, sorry bad picture quality.
The curtains in you car are originals, I have the same thing in different models in o and I gauge. Bing have made some elaborate models with completely equipped sleeping cars with curtains and beds, they are often lost now. Here is another example on a smaller series of cars, end of BING production in 1930 and without markings as they where made for different countries so standard models.
BING has also offered diaphragms for their passenger cars, tey are similar to Marklin ones but I still have to find some.... Sometime they are available in Germany on the Bay.
Very best, Daniel
And here's the next question- how do you get a wheel/axle set out of one of the Bing 1920's O gauge trucks? I'm used to being able to widen up the truck frame enough to slip one end of the axle out (either by some gentle bending or disassembly), but I've discovered you can't do that with these. I removed the side frame from the truck with some further loosening of the tabs (this one was already loose at the old solder joint between the bolster and the side frame), but the ends of the axle appear to be captive inside the little O gauge axle box on the side frames. Do they have a collar on the end of the axle?
I need to straighten one bent axle, hence my interest. Some photos attached, along with a video of the Bing clockwork train running with the other two coaches.