I don't understand your argument? My MBP also rebooted after the security update from Apple 2018-001. Now I toke the English version for the first! but would like to change back later on! Sorry, I had to tell that I'm working on MacOS El Capitain 10.11.6!
Still no German version 6.5.600.2 found! Just tried to download and install the actual version but it was only the 6.5.600.1!