bumping this thread in case any other pre-130513 chassis owners (build date before 05/13) are looking at Koni shocks. After a bit of back-and-forth with a Koni rep I got information that the 8048-1423SPORT rear shocks come with an optional sleeve that adapts the shock bushing to either the bolt configuration (20mm) or to the stud configuration (14mm). as detailed in other peoples' comments the Bilsteins only have a 20mm bushing option for their B4 (maybe you could ask for custom valving rebuild instructions to increase rebound). but for my purposes I need more rear rebound so I think the Konis are the way to go for me.
I researched the hardware kit for the Mazda6’s 1423SPORT which comes with a tubular steel sleeve in the hardware bag with 20mm OD x 14.1 mm ID x 46 mm long dimensions (no drawing available unfortunately). This is exactly the dimension that would be needed to reduce the bottom mounting hole from 20mm down to fit a 14mm bolt. This sure sounds like an adapter to go between the early 20mm Type A and later 14 mm Type B. If your car is the Type A, you simply don’t use that adapter sleeve.