I play a PT1 stencil (Sonora) made in 1986 that has a smaller diameter leadpipe. As a result, I need to use moupieces with a smaller "British" sized shank. I'm not sure as to the origins of the horn, but the wooden anvil case I got with it has a "Boosey and Hawkes" label in it. The case may or may not be original.
I recently sat in with a band who's tuba player had a horn virtually identical to mine, except it was branded Bach, and had a larger leadpipe which could handle a mpc with a larger shank.
These two horns may or may not be from the same manufacturer, but it made me wonder as to why the difference in leadpipe sizes. Is it function, design or tradition? What impact does the diameter of the leadpipe have on the playing characteristics of a horn? Does size really matter?