"this looks like a luck to you because MB_Unit_ID can be set differently with device to device."

SMA set the Unit_ID = 3.  This is non-negotiable and SMA advise this in https://www.sma-sunny.com/en/how-to-test-the-connection-to-your-sma-inverter/ when setting up Modbus coms.  (I found "Modbus Poll" to be a much better tool than Radzio to check registers.)  SMA do very little to support Modbus users and their generator control functions are littered with bugs.   I have been waiting months for a reply and gave up.  I did finally work out how to use an EV as generator input to a Sunny Island, which required a PLC controlled contactor to stop the SMA system autostarting:  Just stopping the EV output generates a generator fault due to the high decay rate of voltage and the local grid crashing... moan, moan.   

"Also Holding register address is usually described inside manual of device so user need look at manual for register addressing as is expected and required."  Yes, this was published by SMA and where I located the register address.  I'll upload if anyone asks since I can no longer find on SMA website.

If anyone requests I will upload my Siemens code - I control a Britwind 9000 turbine with my own automated brake function, Victron EV charger, Grant air source heat pump, Daisy chained RS 485 Modbus Temperatue & Humidity sensors.