Tom,
"Drifter" has the Cape Horn mounted on Bud's stainless steel boomkin. It's likely that the photos that Bud sent were of my boat (previously named "Xapic".
These photos were taken as I just began the restoration, but they might show the details of the vane mounting better than the photos Bud sent you. It appears that the bottom braces were mounted directly to the hull, beneath the "eyeplates".
Since I didn't install the vane initially, I can't really answer your questions with regard to why it was done this way. I believe this was done by Rick Kennerly (two owners back). Rick is easy enough to find on the web. Search for "Rick Kennerly" or "Rick the Mouseherder". I can send you contact info, if you can't find him.
Hope this helps.
Jack
http://westsail.info/images/Drifter_Cape_Horn1.png
http://westsail.info/images/Drifter_Cape_Horn2.png