Pricier as they are made in California rather than Avid in Taiwan, but Paul components does not use a frame mounted spring hole on the post .
He makes the springs within the brake , and by winding up the spring with an open end wrench and tightening the Allen head mounting screw
the return springs are individually adjustable (and sealed inside O rings) It is of the type Francis is referring to..
do all brake bosses designed for cantilever brakes have the same cylindrical housing diameter?
Pretty much, but some variation .. Paul Resolves the discrepancies by supplying a stainless sleeve in the brake design , to control that variation, for his brakes..