Apologies, I have mis-remembered. The foot in "The Resourceful Woodworker" by Wearing, was attached to a
wooden router body:
I hope Batsford won't mind me copying those photos...
Its design was then extended to powered routers via threaded rods and a similar foot mounted on them; you can easily find images of same throughout the net.
AndyT and I refer to a simpler method above, whereby you simply attach an offcut the same thickness as the tenon stock, to the underside of the router. Quicker, less fiddly.
There is a further development of the attached foot, in that, particularly in American 'hand work' circles, they screw on a base with one side exaggerated in length, with a top knob. This gives a greater moment of force at the cutter and make the whole thing more stable, as the lateral arm can rest on the whole length of the tenon stock and the router cannot 'cant'. Sorry, dashing off to Edinburgh on a domestic emergency, so cannot track down a photo right away.
Sam