What is this Antique Brass Instrument on display at the Musical

Does anyone know what type of instrument this is? It looks like some type of Euphonium, but obviously very different.

Size: 20 inch (Length) Material : Brass Finish : Shiny Brass Brass Trumpet with Canvas Cover Handmade, Not an Antique

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What is this Antique Brass Instrument on display at the Musical Instrument Museum in Brussels, Belgium? - Music: Practice & Theory Stack Exchange

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Music to my ears ??? Spotted this old tuba for sale at Arthur

Help me find this new musical instrument, maybe invented in the 70s - Music: Practice & Theory Stack Exchange

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Slide Trumpet in E-flat – Duke University Musical Instrument