It has an eccentric BB but I wasn't quite happy with the chain tension, and didn't have a half link handy, so saw it as a good excuse to use a floating chainring. That sync chain has now stretched a tiny bit (<0.5%) so a whole link could be removed and the eccentric BB would suffice alone.
Anyone looking for a decent tandem in 531, I'm selling mine:
https://www.lfgss.com/conversations/388185/
Project thread here.