About 10 years ago I bought this handy tool from a friend who was a longtime blacksmith and tool collector. He said he had only seen four of the vises with a pivoting rear jaw. I was looking for a heavy vise which I could mount lower than my post vise. It is mounted on a concrete filled barrel at my primary work station. It stands a few inches higher than the Peter Wright anvil and a bit lower than the Trenton-Fisher double screw vise.
Most of the time I use it with the jaws parallel but it does work well for gripping slightly asymmetric things. I really haven’t tried to determine the maximum angle of effective grip.
Of course any parallel jaw vise can be fitted with a jaw insert which will pivot to grip asymmetric items. I made one a long time ago when I was making some soft jaw inserts and inserts with a larger radius edge. I don’t recall ever using it though.
My vise was made by the Reed Manufacturing Company of Erie, PA. Patent dates of 1908, 1912 and 1914 are shown in the casting. The company is still in business making tools including many kinds of vises.
This vise has been most useful because the jaws can open to 8.5” with the slide fully engaged. The rectangular slide surrounds the main screw and not and keeps dirt and debris away from the threads.
I looked up some information about this unusual tool and was pleased to find the author of the article below felt the Reed vises were the best ever made.
Link to an excellent article about vises, their manufactures, restoring vises and more:
More vise discussion (there were 131 pages so far):
Link to another swiveling rear jaw vise - scroll down a bit: