These Loudspeaking Telephones No. 4 are quite rare on UK Ebay, and I've never seen one with the external microphone. I wonder how this one made it to the USA?
http://www.ebay.com/itm/221040417114]Ebay (dead link 2-18-2019 - no image attached )
Bob Freshwater's site on the LST 4 (http://www.britishtelephones.com/lst4.htm)