stage 1's used to be smooth body's, no reservoir, no adjustments
stage 2's were remote reservoir, or piggyback, no adjustments
stage 3's were R/R or P/B with compression OR rebound adjustment only
stage 4's were R/R or P/B with both hi/lo compression and rebound adjustment
"Dune Editions" replaced stage 1's temporarily when they had a marketing deal with an a-arm manufacturer. Now stage 1's are back, and "Dune Editions" are gone.
Now, the stage 2+r's are R/R or P/B with rebound adjustment only, no compression adjustment. These have compression adjustment, no rebound adjustment which is why I was inclined to called them stage 3's.
Technically, stage 3's don't even exist anymore since the stage 5 was released. It's a clusterfuck that one day Elka will stop dicking with.
Myself, as well as other companies still go by Elka's original "grading" system, and just added stage 5's after the 4's since they don't share the same parts as the others.