I beleve mate has a configuration for the screen saver to disable it. Unless it is disabled, the dpms settings will be ignored.

Well that's baffled me. I'm back to square one and it's all running fine. I never needed to execute it as root after all and wonder if perhaps I had a typo in my original script I 'corrected' and got working as root.

My most sincere thanks to all that have helped me resolve this issue.

Thank you.

So, I guess I should have suggested a third possibility. The commands you are trying to use will not be effective until there is an X server running (the computer has gone to a graphics/desktop mode). The boot process and login happens before your X session starts, so there is no $DISPLAY for your xset program to find when run from rc.local. You seem to have some scheme (mate?) that I've never heard of, but which seem to run some defined set of scripts or commands in response to some event that occurs after the X server is started.
This hypothesis is consistent with the testing phase, where all of your tests are performed post-startup, when everything has booted and the X session is already active.

--- rod.