Well, I seem to be getting in the habit of looking out for odd aviation-themed items and so was happy to get my hands on this little gem. The TU144 was a Russian competitor to the Concorde. A supersonic passenger aircraft although it didn't have quite such a successful career as the European jet. Of the 17 aircraft built, two crashed and only two went into passenger service. Between them, they flew a grand total of 55 passenger flights. This letter was onboard the first of those passenger-carrying flights, on the 1st of November 1977, between Moscow and Alma-Ata.
You can see the flight and my little envelope take off in this YouTube clip
