I've got a bass from a friend and I'm looking for more information about it. It just has SDGR on the headstock. No numbers referring to a model. I looked up the serial number and it was made in Japan in 1991. That is all I could find. Nothing to explain the control pots. Web searches always brings Ibanez basses with other model numbers but nothing with just SDGR on the headstock. Any info would be much appreciated. Thanks in advance.