Vintage Trumpet? Trombonium ? What is it ?

I inherited this and no idea what is is ! It’s all there with mouthpiece, music stand etc .. lovely blue velvet lining .. case is in good order .. huge thing.. has original manual .. needs a new home .. needs a polish . Silver colour .. no damage that I can see .. the wee keys move well .. made by King .. over 50 years old I think .. will measure and add shipping costs .. love to know what it is ..