It's 3,5mm and comes with a 6,5mm adapter. Cable is 1,8m long. I don't want and don't need longer cable than that

And adapter to 6,5mm is fine , too. Because i have a DX3Pro+ DAC with 3,5mm plugin.
It got in the Outlet Store but was never used by a customer and returned.
It's not the first time i noticed that in the Sennheiser Outlet Store. Sometimes they are customer returns with very minor marks or the are completly new.
MY HD660S1 (don't have them anylonger) were completly new and my HD599SE a return in almost new condition.
I have very good experience with their Outlet Store!
What a steal for 69,99€