Hi, Phoneman!
I had to look up the Williams sight mounting fixture in Brownells to remind me of which one I have. That's the one with sliding parallels that always centers the drill bushings on the receiver, right?
I've used mine for several years now, with an average drill press and am perfectly happy with it. I found it on sale among a retired gunsmith's tools, and chose between it and the Forster fixture which was more than twice as much. The unit locates sight mounting holes as accurately as if I was using a milling machine. Excellent performance for a modestly priced fixture.