I'm sorry, Jonny... I've been so busy that I haven't been on the forum, and I didn't think to subscribe to the thread.
Although I couldn't find a think through a Google search, your comment about "finding your Bill Belcher self steering book" made me realize that I just might have that book myself. Mine was actually, "Self-Steering Under Sail: Autopilots and Wind-Steering Systems" by Peter Christian Forthmann. I wonder if that's the one you were thinking of.
At any rate, created a PDF of the 5 or so pages that made reference to it. There were actually some good photos. I'm sending it to you via email. I hope it helps. When I catch up with myself, I'll give you a call and see if there's any way I can assist... it looks like the vane itself is a fairly simple design to duplicate.
Jack