Where is the "Letter to NBS, October 22, 1975" Discussions of the weakness of the U.S. Data Encryption Standard frequently cite a letter by Martin Hellman and Whitfield Diffie to the National Bureau of Standards that reported stated:

Whit Diffie and I have become concerned that the proposed data encryption standard, while probably secure against commercial assault, may be extremely vulnerable to attack by an intelligence organization&quot; (letter to NBS, October 22, 1975).

I've been trying to find a scan of the original letter, but all I can find is the above quotation, which seems to have been faithfully repeated in many publications (for example, here).
Interestingly, this book attributes the letter solely to Martin Hellman.
Where can I find the original letter?
This letter, among other letters of the time, can be found in the Stanford digital repository of Hellman's papers.
This October 22 1975 letter, in particular, is here.