Haven't read the text in German, but maybe it's cast just for the prototype and once he has got all the dimensions just how he wants them he will fix the design and then build it in a conventional way from sheet metal.
I'm with Rhy's. LaSax company did like that when they constructed thier straight altos and tenors. The took the tube of a Martin saxes and took of the tonholes so they could "move the toneholes around" on the tube. Later the horns were manufactoried on Tawain with modern technique. Drawn toneholes. There is no need nowadays for having a sax built with soldered on toneholes. But why are they not manufactoring saxes with solid nickelsilver keys? This is really good. Durable and sturdy. The Martin I have with nickelsilver keys are better than the Martins without. I think it gives the sax better playability.