Has been something of a wanderer.

Is well known to union men and Socialists, all over the country.

A powerful speaker, and resourceful.
He was last employed at your testing-works on Staten Island.
Discharged by your Mr.Herzog, about two weeks ago for having, I understand, been in possession of a certain red-covered note-book, which Mr.Herzog found in his pocket.

This book is the same which you commissioned me to find, but which Mr.Herzog returned to you before I undertook the search for it.

The inference is that this Armstrong is in possession of some private information about your work, which may make him even more dangerous.