Sadly I fear it has gone the way of all flesh... or rather paper, into the wastebasket whence no traveller returns. I bought all Nico's material back in 1978, but such a letter was not among it, otherwise I would have scanned it (before giving it to GOSH in 2004). It might be worth checking with Beinecke if by some chance they've acquired it by some other route, but I sincerely doubt it ...