Supermicro MCP-290-00053-0N are the rails you want for full extension. They will have the yellow labels vs the old blue labels that didn't fully extend. I have 7 sets of this exact rail so I can guarantee with 100% certainty they are the ones your looking for. That is of course if you have this style of chassis. Some of the really old chassis had a much wider or taller pattern.sorry, don't mean to revive an old thread, but what was the conclusion? i'm looking for rails for 846 too...