I had the same question as have G12T-75'S with the same label. Here is what Dr. Decibel at Celestion had to say.
"The black strip labels were fairly short lived and fell between a change from the original silver label around 1989/90 to us getting our own label printing machine around 1992. ' Mine are 1990 and sound excellent!! Hope this helps.
The story i remember i think i read it around here was that there was a fire or something at some factory where the speakers were made, or they were moving to a different building or something and these new lables were when they were changing over?