Still wanting to buy. Please check your junk drawer and see if you can find one.

The mouthpiece shaft measurements are .50 at the end and .57 about 2/3 up the shaft.

Here are a few pictures of my Buescher Eb. The S/N places it at 1929. The goose neck has a thumb screw to tighten the tuning bit, so I think the bit needs to be a non tapered shaft. I also need a thumb screw too.

I don't seem to be able to private message you, so maybe you could private message me please.
Jeff

WTB Tuning bit for Buescher silver plated Eb sousaphone.