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Have a 110 year old Buescher Eb helicon, looking for a Buescher tuning bit for it. No guqrantee that the neck on it now was original, in any case a Conn bit fits but sits awkardly. I think Buescher bits were a unique zig-zag/lighting bolt looking affair, not the simple slight angle of most sousaphone tuning bits. If anybody has one we can break out the calipers and verify dimensions.

I have a "zig-zag" bit, but BOTH ENDS are BASS TROMBONE shank (UNDERSIZE).

My guess is that it was oem for one of those tiny 5/8" bore E-flat sousaphones.

Trust me: NOT STANDARD SHANK...SMALL BORE !!!

YET: "zig-zag".

Also interested in just knowing what buescher bits were like, myself...

And do BBb have different set ups (neck & bit sizes) than Eb?

As for what Buescher bits looked like, someone posted a picture of some repair parts catalog that identified various bits, no idea where to find that picture now.p, but I recall the buescher having a unique shape.

Bloke, got out the calipers. Bore of the horn is 0.609 or so. The neck is probably a Frankenstein creation of the guy a bought it from, he’s a HVAC and refrigeration guy so he can get creative with a torch. The bore of the mouthpiece receiver is like 0.55 or so, a conn bit goes in about half way.
Picture for reference
https://photos.app.goo.gl/p2z3owcH7MSQosZu6

Looking at this thing again the answer may be to remove that bit of tubing past the last ferrule. If your bit is bass bone sized it won’t help all my mouthpieces are standard shank.

Your gooseneck is an altered "new style" King sousaphone gooseneck.