Yeah, I have this one
SKU 4535804.
The channel that the o-ring fits is a stretch fit and the o-ring that came on it was nearly too small. I replaced it with a 6mm x 1mm o-ring which is perfect (less stretch). It was from a GS II. They are both normal o-rings.
It is a very nice vape, but it a heavy beast. :P
I think the o-ring in the set is the 5mm red one. I haven't purchased it. They are wish listed.
I tried to order a replacement tank for this, but FT sent me the wrong one and they haven't confirmed that they have corrected that issue so I don't know how much more I will invest into it. Having said that though, they are cheap enough to buy as a complete atty.