One question that comes in periodically is about a type of double horn that seems to have been constructed to have the valve slides in the wrong places.
I first ran into one of these back when I was in high school and indeed they do look a little odd. The first one I saw was an old Sansone, but you are just as likely to run into an old Holton, their “pre-Farkas” model.
For example this instrument, the photo being linked from the very interesting horn-u-copia site. Look closely, the Bb horn valve slides are “on top,” they are in the slots where normally you put the F horn valve slides. And the F horn slides are “below” in the place where normally the Bb valve slides are. I think the general idea was that this design was easier to get the water out of the valve slides.
It is somewhat arbitrary which “side” is built as the top, but we all know that the F side is usually on top, for aesthetics.
If you put the slides in “correctly” for a standard double horn these horns won’t play remotely in tune. If you are testing a double horn and this is the case, consider that it might be one of these vintage horns.
To see what you have if you are not sure, pull out for example the first valve slides, push down the first valve, and see where the air comes out with the thumb up and down. If it is this design it will be clear pretty quickly.
Perhaps some maker will revive this design? It would stand out from the crowd and, in terms of water removal, it is not necessarily a bad idea.