Close-up of Shoulder U-Joint. The headless bolt
threads into the hub (yep, Phil Wood hubs thread
into them, counter to what you'd think). The other end
of the bolt is threaded into the U-Joint nut and further
until it tightens against the central U-Joint block. This
is what constrains one axis of U-Joint spin (vertical
in this picture). The other axis of rotation is visible
head-on in the circular ring dead-center. This axis of
rotation aligns, by design, with the servo axis of rotation.
A little more filing needs to be done so that the U-Joint
will rotate to point straight down (and up).