I'm not 100% but imagine it relates to the different lengths of straddle cable, which are also marked A and B. Sheldon Brown has a page on this https://www.sheldonbrown.com/cantilever-geometry.html.
Thanks SteveNice work - looks great! The Autotek stuff's good - the primer went on and seemed to stick on really well for me.
Cheers dudeLooking great. Can't wait to see the final product.