Missing letter from Arthur C. Clarke

An announcement forwarded by SFScope friend Andrew Porter:
From: “J.B. Post”
Date: July 11, 2012
Subject: Stolen A.C. Clarke letter
Back in the ancient days of my involvement with the Philadelphia Science Fiction Society, I was the program planner for the annual PhilCon for a few years. One year I asked Arthur C. Clarke to be the Principal Speaker (there were no Guests of Honor in those days at PhilCons). He declined, in an air letter, saying he would be involved in working on a movie. We got Asimov instead.
Anyway, a few years later when the air letter was displayed at a meeting, it vanished. It might show up for sale sometime if it still exists. There is a good chance it has been destroyed, perhaps by the thief in a fit of guilt and fear of disgrace, or perhaps by the heirs, deeming it worthless crap.
Such an air letter addressed to me is the rightful property of PSFS. If it has settled in a collection of Clarke material in some institution, I don’t think PSFS would want it back, merely a note that it is on loan would allow the institution to keep physical custody.
Thanks for anything you can do to get the info on the Clarke letter spread around.
J