I got this at recycle centre. It appears to be a 3-phase rectifier bridge. But strange enough I could not find the datasheets for the two parts S20ZND06 and SC8K250ZD. Both have 3 poles each, +/~/- for S20ZND06, and K/C/A for SC8K250ZD. I believe S20ZND06 is a dual diode module. No idea what K/C/A are, though K or C probably Cathode and A Anode.
Also intriguing is that each pair of S20ZND06 and SC8K250ZD are connected in parallel +/K, ~/C, and -/C, which makes me thing C probably stands for "Common". Easy to understand identical parts in parallel to have higher capacity or rating (though sometimes not that simple) but here two parts are not the same.