I always thought copper was preferred. Why do you say but?
I personally prefer copper mouths, the "but" was meant in case she was looking for a plain stainless mouth, in which case I have yet to come across a double extension that is just "plain."
I've got a bunch of leather covered bits in the barn...mostly ancient....they are basically decorative at this point, just hanging around on hooks. I have no use for them, maybe a rainy day craft project. The leather gets nasty pretty quick, roughed up and broken, seems to cause more harm than good.
We joke around that 90% of the horses in the barn could be driven with just a halter and baling twine for lines....but then again, most of these old guys would get around the track with a sack of grain on the seat. Haha.