Again thanks for your great questions! Pardon the randomness of my replies!
The horn's outer slides are nickel trim to add a bit more definition to the attack. We tried nickel silver mouthpiece receivers but did not like the results.
The mouthpipe is primarily composed of yellow brass but has 5% more copper content to help with any potential wear issues. We didn't the acoustic result with anything over that in terms of the additional copper % in the pipe.
We chose this bore as it is a bit smaller than the standard compensating euph as we knew the primary market for these instruments will be either students getting their first euphonium, moving from their small shank baritone, or trombonists looking for an affordable option to double (hence our doubler name). We hope that makes it an easier transition.
We are still considering doing something similar to the Yamaha 321 but I don't really see it in terms of the overall concept of what our doubler line is. In feel it's closest to a 967 with YB bell. Of course it's not going to have the refinement in the sound like an Adams, Besson, etc (we do sell both of those brands at the shop of course). With the bore we do the same on our high brass instruments with good success so our first factory order will be this bore. We can always change it in the future after some time on the marketplace and reviews.
The guides are plastic but I'm working on a metal option in the future although it's much louder in terms of the action and less smooth overall in the feel as well.
I've never been asked about the plating thickness so I'll ask the factory. It's robust on our high brass but I do not know the exact thickness.
Best,
T