One would have thought the max voltage would be the upper limit of the VP solar kit, 6V battery but does output up to around 6.8v
The official word from Davis is not to exceed 6v. Slightly higher voltages can certainly be tolerated but the higher the voltage and the longer the exposure then the life of the regulator will suffer. I can imagine that 6.5v (or maybe even 6.8v) is Ok for limited periods but you wouldn't want to be using eg continuous connection to a 7.5v mains adapter for more than brief periods.
Well nobody mentioned 7.5v mains adapter but if a Davis console or ISS can not handle a continuous power feed from a Davis solar kit then somebody has got it wrong, so that users don't get confused here we can say with confidence 6.8v is ok, it simply has to be because that is what the Davis solar kit puts out.
I wasn't able to find the specs on the solar kit that (apparently) quote the 6.8V output voltage. However, being that it relies on solar power, this is much more likely to be an absolute maximum figure under ideal conditions than a continuous rating. There is a substantial difference, particularly in terms of potential thermal stress on the regulator circuitry, between occasionally being able to handle a
peak power supply voltage of 6.8V and being able to handle a
continuous input voltage of that level.