Got 'Authentication Required'

Google found me this. Least seedy looking source I could find.

http://www.globalsecurity.org/intell/library/policy/army/fm/...

Worked for me just now (7 minutes after your post).

Minor general complaint to people tarring up stuff for download; It's always useful to provide a useful title and/or author to identify the file, instead of something like 'pdf.tar.gz' or something equally distinguished.

EDIT: I'm not wanting to sound ungrateful, it can just be a little difficult to notice or remember to fix a name, and then... hunt the file time ;)

3yo https://archive.is/oM5Q (only the index page is archived, the links will redirect to the originals, if you had trouble getting the index, they might be blocked too)

Weird, still works for me (or works again).

But yes, same material as your source.

http://www.und.nodak.edu/org/crypto/crypto/army.field.manual... contains zipped pdfs for each chapter.

I really like that this includes frequency tables for tetragraphs.

I had to laugh a little at the fact that "STOP" is the 5th most frequently used tetragraph though.