I read somewhere that these sets are pretty rare, because of the 2-line switch and exclusion plunger. Well, I saw the listing for $74 BiN, and by the time I clicked on it, it was gone! :o
http://www.ebay.com/itm/171780617270
I have one of these from 1960 (711B), except that it has something of a toggle switch connected to the chassis instead of a knob. I also believe it was never used. (guy I bought it from says he still has the original box!) Would that be any rarer than this?
Here is a 711XB with a 664A transmitter added for use with the Home Interphone:
http://www.ebay.com/itm/151643295601
Here are descriptions for the 713B sets