What I've never seen before today, and this is at a new firehouse, is a stipulation that the fire retardant stamp/label be visible on the backboard. I've never had to label a backboard as such. I searched, but could not find a roll of labels for this purpose, and I would think one would have to show some type of certification to even have possession of these.
There is no such thing as fire retardant labels that you stick on the backboard.
What is required is the fire retardant is applied to the backboard at a factory and a ink type stamp is applied at the factory that marks them fire retardant.
As was said before. When you paint these backboards you have to mask off the label(s) so they can be seen by an inspector. Don't get caught in the trap of making off the label and then covering it with equipment.
