Today I received a weird envelope if you can call it that.
It is a air mail envelope but the thing is it was one you had to fold yourself. It had a pre printed 11 cent stamp on it already. I have gotten a few air mail stamps but this was the first time I have gotten a envelope. It came from maine.
I had bought some $1 stamps and that is what the seller decided to use to get them to me.
On the bottom it has AEROGRAMME PAR AVION so I am thinking it might be french or mexican.
What you have is an aerogramme or air letter sheet. They are not envelopes, per se, since enclosures are not permitted (when they were current). The sender would write a message on the sheet, fold it per the directions, and send it on its way.
In your case, the sender used it as an envelope. I'm not sure if U.S. postal regulations allow that. But, it went through the mail anyway.
Air letter sheets are found in the Scott U.S. Specialized catalog. The catalog number is prefixed with "UC".
From the USPS's International Mail Manual, Issue 8, dated June 28, 1990:
I suspect the aerogram regulations were quite similar at other times before and after this publication, but it is a starting point for reference.
Reading the last paragraph backwards ... it seems aerograms could have enclosures if uprated to the full letter rate and would pass on naturally through the system without any delay, penalty, or postage due.
PostalHysteria, Yes, I understood your question also. My response was directed more at the previous comments. That said, I know of no prohibition, which would be in effect a demonetization of them. With today's automation, anything tagged will fly through the machines and end up in a delivery-sequenced tray ready for a carrier to deliver before any postal employee ever sets eyes on it. Common sense should come into play to send merchandise lots enclosed in paper thicker than an aerogram.