Serial numbers definitely indicate that they weren't built by Keilwerth.

Keilwerth made at least a couple different versions of the H-Couf Royalist models for Armstrong, but they didn't make all of them. I've seen at least two different models of Grassi horns and I'm sure there are others.

There is a model made immediately after the "Model 28" Royalist that has "Armstrong" engraved on the bell and no mention of H-Couf. These horns have serial numbers that start with 29.