I am not quite sure what this is. Possibly an Alto Flugelhorn, or maybe an "Althorn in Trompetenform", or a Solo Alto Horn? This instrument looks very similar (apart from the much larger bell) to an "Althorn in Trompetenform" by Edmund Paulus from Markneukirchen ( https://brasstacks.de/edmund-paulus.html ), but there also is a Bass Trumpet by Moritz in the Horn-u-copia Catalog that looks very similar to this horn ( https://www.horn-u-copia.net/instruments/Moritz/GW-Moritz-Berlin-Fl.jpg )...

The horn is faintly but unmistakably marked "D. Ansingh & Co. Zwolle". However, Ansingh didn't make their own instruments but sold stencils, mainly from Bohemian makers. So the real maker remains unknown.

The mouthpiece receiver is very wide, it fits a large shank trombone/euphonium mouthpiece. It plays in E-Flat (at least around there, depending on the mouthpiece).
You can listen to me play it here: https://www.youtube.com/watch?v=25N1rnqa-jQ

Unfortunately, shortly after making this video, the first valve went from bad to worse and it is now unplayable.