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This one is in Japan and has been up for a while. What distinguishes the 3107 from more common B&S models like the 3101 (i.e. the 101) and 3103? The only info on the model seems to be on some Japanese sites. It looks like a slightly larger 103.

https://www.ebay.com/itm/285153370818

These tubas all have the same bore, but a few differences in bell diameters. The 3107 seems most closely related to the 103, but I have no idea what the actual difference is.

Bell diameters:
103 = 450mm
3107 = 450mm
3103 = 480mm

The 3101 is a bit different, it is taller and has an even smaller bell at 420mm

The picture of the bell looks as if it is 80/20 brass, but that could simply be the type of light source affecting the hue.

I don't know why I'm even posting this, because I'm not one who much cares about different brass alloys.

bloke wrote: ↑Tue Mar 07, 2023 10:28 amI don't know why I'm even posting this, because I'm not one who much cares about different brass alloys.

…I think you care somewhat when it comes to corrosion resistance, which is real and observable.