PPIE, The Panama Pacific International Exibition was held in San Francisco in 1915. Stamps for the PPIE were first issued 1/1/1913, 1, 2, 5 & 10 cents later a different color 10 and later still the 1-10 in a different perforation.
Here is the back of a PSE.

This contained a two page, one sheet letter marked "copy."


It seems to indicate that a gentleman from Berkeley, CA heard someone could use some cash to get someone or something to Minden, NV. which is about 28 miles north of Topaz, CA. He confirmed he was sending $10 cash.
Now for collectors of PPIE originating postal history what tends to be looked for and collected are covers with the PPIE stamps which also have the PPIE perfin plus on an envelope with a corner card that has some association with the PPIE and lastly the entire combination being sent from the USPOD's Model Post Office at the PPIE. Such is an example:


Now if one was a coin collector who covers the coinage issued at and for the PPIE, the sender's name is quite well known. I merely described this cover to a stamp and coin collector at one of my clubs and he said, don't show it to me before I tell you who sent it. He was correct but also fell in love with the cover.
Now as beautiful of a piece of PPIE postal history as this is, there is one item missing. Of course this item is always separated from the mailing and rarely saved after a short period of time.
Back to the envelope back I first showed, below is the PSE front in a condition many would find lacking. Except that in addition to the letter it contained, it contained the registry receipt for the $10 cash sent by registered mail from the man in Berkeley to the man in Topaz. That receipt in full size is shown below the envelope.

And now a close up of the cancellation.

Happily the man from Berkeley decided to send the $10 cash when attending the PPIE himself and did so from the Model Post Office. The letter itself is likely long gone but a very scare PPIE Model Post Office Registered Letter Receipt remains.
What is quite amazing is the PSE and letter were saved at all given the poor condition many collectors would consider it is in. Of course the gem was not the PSE, but the registry receipt to which the letter and PSE added a nice polish of a story for the gem.
If there is a moral here, don't judge the contents by the condition of its wrapper.
So why don't we use this thread to show YOUR interesting PPIE covers and not usually seen postal documents with the PPIE Model Post Office cancellation.Edited for English, title, new first paragraph and emphasis in last paragraph.