Lucky me to find a perfectly working 6579 just sitting in the garbage.
But how to get that nasty intrusion alarm to be quiet as I wished to
run the unit headless?

I finally figured it out - be holding down the power button when you
plug the unit in. The machine will power up for a second or two then
power off. When you next hit the power button you will discover the
intrusion alarm is now cleared.

cheers.