They duplicate the contours of "other" brands such as Bach and Yamaha and the Helleberg tuba/contra piece... and as such, the pitch should be no different. Shank length differences could do that, but not the material a mouthpiece is made of. Read Mr. Schilke's clinic paper on the Schilkemusic.com site in "Archives". Paper discusses the physics of horns, mouthpieces and your mouth cavity.
I know the guy that makes these. He's 15 minutes up the road from me in Fond du Lac. Terrific guy. A euphonium player that owns a precision machine shop. Found a way to combine his career and hobby. Good for him!!!
A friend with Illinois Brass Band tried one of my samples (5C), and his only negative comment was that at certain frequencies - pitches, he got a sort of vibratory feedback from it, as it has much less mass than a metal mouthpiece (part of the supposed physics behind "megatone" slugs). Everything has a resonating frequency, you know, but more mass resists the resonating. Or whatever. I'm just an artist. He sounded great on the 5C on his vintage Bach Strad. I have one, but still prefer my special little monster. Pretty soon he'll be able to cut custom ones, and I'll be in line for a plastic version of a Schilke 58E2d.
These things are great for cold weather -- and don't get too hot in the sun on the field (not sure about the black ones -- there is a Kiltie contra playing one... I'll ask.)
