Perhaps this subject has been discussed, if so I'm sorry.
In most footage we have from Queen live (even back in VHS era) it is visible some sort of BANNER stage-left, almost near an stage exit, posted to the wall, saying what it looks to be "DUDS".
Does anyone knows what that's about?
Duds are the microphones/bulbs etc which do not work.
The word duds is plural of dud.
A dud is defined as: link
A bomb, shell, or explosive round that fails to detonate.
Informal. One that is disappointingly ineffective or unsuccessful.
Therefore, it is a box for the electrical eqipment which does not work - and will be replaced, repaired or disposed of later.