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 at least one Borgani model. I'm sure there are others.

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