This page contains everything I know about the Mumford link, I know of no further resources beyond what I have collected here.
Click here to search the Eng-Tips Forum for more info. Here are the three I found in Feb 2005, thread 99363, thread 71854 and thread 35203.
The Mumford link live axle is a refinement of the Watts link. It has the ability to control the roll center as well as a Watts link, but has the ability to move it much lower than with a Watts (the roll center on a Watts is through the pivot bolt, whereas on the Mumford it is at the theoretical interesection of the two link arms). Here's a scan of an article describing the Mumford link; I believe that it is from Racecar Engineering V3 #5, in an article by Arthur Mallock.