You need to visit our "Value your horn here" section on the website for a full reply, but right off of the bat I can tell you that the "HP" stamped on the horn is an indication that it was produced back in the day of the dual pitch system.
All current instruments are produced to what is called the "Low Pitch" (A=440 hz). Previously, a higher pitch ("High Pitch", where A=475 hz or something like that) was also in use, and instruments were produced for both systems by most of the manufacturers. In the old days (i.e., back when Pete was still able to get around without a walker), all instruments were stamped with either "HP" or "LP", this to avoid confusion.
(My grandfather, who brought his clarinets over from the old country after World War I, had sets of both Low Pitch and High Pitch instruments. When I was learning how to play the bass clarinet, I first started on his "HP" bass, and only when I proved myself capable of careful handling of the instruments, was I allowed to switch over to the "LP" bass. That it was a Buffet Albert system bass clarinet pitched in A only adds to the complexity of my early music story...)
A High Pitch instrument is of little use in ensemble situations, since it is impossible for other woodwind and brass instruments (all produced to the Low Pitch specification) to tune to the High Pitch specification. Even a perfect example of a given instrument in High Pitch is only useful as a historic curiosity, and thus would have little value for anyone wanting to play an instrument in a group situation.
If you decide to sell the instrument, make sure that you specify that it is "High Pitch" in the auction; otherwise you will run into difficulty when the buyer finds out that the horn is useless other than as a museum piece. Caveat vendor, so to speak...