Del Val Miata Club

I purchased a '91 Miata which is in great condition w/ 15K mis. on it.  It has what appears to be a "SUPERCHARGER" insignia on the back of the car that looks original.  I was told that it is a factory installed supercharger when I purchased it.  Someone on this forum questioned that validity and said "no such thing!"  Can anyone comment about whether was a factory installation?  I Googled it and can't find anything.  Thanks