Here is the link to the archived PDF. It does have ",...un.gov\,.." within the URL but am unsure that this is a "proof" that it is a valid UN Letter.
I'm not saying it's not, I believe it is, but am thinking what a jw apologist would argue. Better to be prepared than surprised.
http://web.archive.org/web/20090126152929/http://un.org/dpi/ngosection/pdfs/watchtower.pdf
NJY