As I said before, Raimey, I just don't know, GG doesn't know and IMHO you don't know either, so we are all on the same page.
So far all we got are hypotheses...
There is nothing wrong with that...Even though it's not satisfying.
The only thing we know is that what works for "ZSR" should work for "ZS2" as well.
That should keep us busy for a while!
All the best,
WC-