As I wrote in the past in the Netherlands HP instruments have been used by marching bands up until the '70 ans I have found ( and unwittingly bought) Amati ( they have a small H at the back of the horn) and Dolnet ( no designation!) in HP , I also once found a Couesnon that was shorter than normal and that wouldn't play in tune, nothing identified as HP. So, it is not only Conn, when buying a horn always measure it ( High pitch horns are shorter) and if possible play it with a tuner. You can tune one note but the mouthpiece would be almost falling off to do so and the other notes wont' be in tune or, as it were, the horn is not in tune with itself.Some people claim that they can play, especially HP sopranos in tune. This is possible with great reed control and bending the notes but it is almost sure that the tuning will be , at best, approximate.
In this video the owner and player claims he can do it, in my opinion he can't and fools himself by using lots of vibrato and changing rapidly the sound.
If you would put long notes against a tuner it would show a different story.