You're right!! It's simply:
Open E tuned down past open D to open C..!!
I use this tuning because I like a lot of low end, it gives a fat bass, very nice when you play solo or duo without a bassguitar involved..
And it gives less tension on the vintage instruments.
Also, for me it feels better to play, easier to fret without wearing your hands out in a 3 hour gig, haha!
I use
0.15 - 0.58 in this tuning also for open F which I mostly use these days, like open G tuned a step down.. (On my Resolian I have a 0.62 bass string)
But I'm just as comfortable playing and singing in open A, G, D and E!!