German or Austrian Post Horn
Signal horns are well known in many ancient cultures and were
used as early as Roman times to announce the arrival of the
emperor's messengers.  The European tradition of regular
postal coaches was firmly established during and after the 16th
century.  By the late 18th century, the German Post horn is
standardized in this shape and pitched somewhere near modern
Bb.  The calls required just two or three tones but the desire to
become more musical resulted in a longer tube pitched in F or
Eb, often with a transposing finger hole.  In the mid-range, this
raises the pitch by a fourth or to Bb (on the F instrument).  
The combination of the F and Bb harmonic series allows for a
diatonic scale in that octave, giving the postilion the ability to
play many folk tunes along with the simple calls.  My example
pictured in the first three images to the left, pitched in F with 4
1/2 turns in the tubing, is the form found commonly
throughout the 19th century.  Although no maker's mark is
present, it is a deluxe instrument being made of German silver
with gold brass trim.  I believe that the mouthpiece is original
to this instrument and plays quite well.  The bell rim diameter
is 4 9/16" and maximum length from bell rim to mouthpiece
receiver is 10".   I have been told that a deluxe post horn such
as this is probably from the 1860s and is called a "Bavarian
post horn of honour".  These were awarded to postilions for
service or playing abilities.
 An identical instrument was
appraised on Bavarian television's "Kunst und Krempel", a
series very much like the British and American "Antiques
Roadshow".  That instrument had belonged to the postilion to
Kaiser William I before 1885.  I invite you to contact me if you
have any additional
information that you might be able to offer.

In the early 19th century, post horns were also made with as
many as five keys or with two or three valves as pictured
below.  The example with double piston valves is on display at
Boston Museum of Fine Arts and still has no tuning slide
since is was probably not intended to play in a group of
musicians.  The last photo is the form most commonly found
today and is tunable in order to play in bands and orchestras.  
Most often these are pitched in Bb, the same as a cornet, but
this example and others that I have seen are pitched in C.  I
have not learned for sure, but my speculation is that these are
intended for use in the parlor or church, playing along with the
piano, organ, choir etc.  It is normally accepted  belief that the
French cornet-a-pistons and thus all modern cornets and
trumpets grew from these origins.

An instrument that is commonly confused with the Germanic
post horn is the German hunting horn or jagdhorn.  These are
also pitched in Bb, roughly the same shape and modern
examples are available with or without valves.  They are
traditionally wrapped in green leather and have a name plate
with "Fürst Pless" embossed.  This signifies the origins under
the late 19th century Prussian rein of Hans Heinrich XI, the
Fürst von Pless or Prince of the duchy of Pless in Upper
Silesia, a region of Poland today.  The principal difference
from the post horn is that they have a faster/larger taper and
flare, ending in a bell more like a flugelhorn than a cornet.  
These are a common choice for performing the post horn parts
in the orchestra when called for by Mozart, Mahler and
others.  Another fun variation is the taschenjadghorn or pocket
hunting horn, still in Bb, it is coiled so tightly with a much
smaller bore, it will fit in your jacket or coat pocket.
Click on images for larger views.