I found another of these mirrors years ago, which is what made me know to flip it over, see the Bell System type on the back, and grab it. These mirrors were issued to linemen who worked for Bell Telephone, to use inspecting the hidden undersides of telephone cable splices. I love picturing that, and also love the object-hood of the mirror itself: heavy and substantial, not an easy one to loose track of, soldered at the corners and with a great untouched patina to the brass now.  The slightly wonky pressed type lettering on the back is really excellent too. 

3 1/16" x 3 1/16" x 1/4". Very good condition, terrific, untouched patina, c. 1920s-30s.