I looked this windvane
as I too am interested in a Norvane. sizing has to do with distance from the water
to the deck
...basically your freeboard. I did some checking.... at or close to his asking price
you'd pay more once you had the proper brackets made.
size 12 on Norvane's site is 1985.00 and spare kit is 195.00.
I hope OP does sell it but I believe if it doesnt mount up to your boat
with his brackets you'd spend more than buying
new getting it to fit your boat