Neurath and Russell

So I'm reading Otto Neurath's "Universal Jargon and Terminology" (you know, like you do), and I see this on the first page:

"Bertrand Russell presented his classic thesaurus of symbolic tools;"

What text do you think Neurath has in mind?  (I didn't see any helpful/ illuminating context around this bit.)

If Neurath means the Principia, I think that would be hilarious; the last word I would use to describe that book is 'thesaurus'...