I've seen this wrench in pictures but never in the "wild". I have a soft spot for quick adjusts, so this one followed me home.
It is marked "Bay State Tool Co., Boston" on one side, and the patent date of June 17, 1904 on the other.
The operation is a little unusual in that it takes two hands to easily adjust it. It's doable with one, but much easier with two. As you can see, it has a screw like most other monkey wrenches but no adjusting nut.
In this design the threads are milled off lengthwise on opposite sides of the threaded rod, then when the spring return lever at the end of the rod is moved 90° the threads are disengaged inside the moveable jaw, and it can then be slid up or down the rod. This must be done with your other hand.
Releasing the lever allows the threads to reengage inside the moveable jaw. It's actually much easier to do than it is to explain. The link below will take you to the DATAMP site.
http://www.datamp.org/patents/advance.php?pn=761798&id=15720&set=2Mike
