1956 Nathan Airchime Prototype P1 "109"
Here are a few images of what I think is the earliest P horn in existence. Now, I can't prove this fact since there is no serial number stamp on it, but there is a 109 stamped along the lip of the "power chamber." I can't tell if the "109" stamped into the top is a serial number or not, but it does use a similar font as the date stamps on the back of some M horns.
It has an aluminum backcap with no "Nathan" letters cast into it, an oval inlet, and a beryllium bronze diaphragm. The square manifold base is another rarity, with the "Pat Pending" cast into the side of it, and a very small restrictor in the top of it.
Another thing worth noting is that it is 17 3/4", while my ghost oval P#1 bell is 18 1/2" long (diaphragm seat to end of bell). It is visibly shorter. I ran a sound file through audacity and it plays 284 hz, between a NC and OC P#1 bell.