A while back I had a question come in on the 5-valve single B-flat horn. These are still made but they are not common and were somewhat more common back in the day. As to how they sound, I have tried horns of this type and they have a lot of the sound of a double horn with the agility of a single B-flat, but in actual use might still stick out a bit in a section of double horns.
The “classic” model of this is certainly the Sansone model, developed by Lorenzo Sansone. This illustration is from the Kruspe 1930 catalog, more images from which may be accessed from this article.
What made me think of this question and this being a good time to reply briefly is the most recent video in a series put out by Siegfried’s Call is on a vintage Sansone horn of this model that they have for sale. I don’t see a way to embed the video in Horn Matters, but these instruments are seen occasionally and an interesting footnote on horns used professionally in the USA in the early 20th century.