I know that hanging a US flag upside down is supposed to be a subtle way to tell people something is wrong. They would do it all the time in war movies to alert troops not to come back to the base because it had been overtaken by the enemy and they were hiding in ambush.
My WAG is: a stamp upside might mean something to the effect of ‘everything in this letter (or whatever) is bullshit and you should disregard it’ Or, everything in this letter is the exact opposite of what it says.
Incidentally, when I was a younger lad, since the stamp cancellation process is automated, we’d put the stamp just to the right or left of the ‘send to’ address in roughly the middle of the envelope which casued the cancellation machine to miss it and you could reuse the stamp. Also, if you think ahead, and are sending a big package requiring loads of stamps, cover them with white glue (elmer’s), and when it dries, it forms a clear coating that allows the cancellation ink to be wiped right off.
I know, you have to be pretty desperate and cheap to be re-using stamps, but we were poor college students at the time.