20 yrs is a very long time. LARGE PD and sheriff's dept might computerize items description but back then it was very likely on a diskette and a DOS machine. Most likely it would be on paper and in storage (if it still is available).
You possibly need an "inside track" who is willing to go the extra mile for you AND you may need to open a whole new report with pictures, receipt, serial #, and another individual willing to verify the theft (if no store still exists, etc). Po;ice reports on non-felony thefts 20 yrs back are often VERY tough to get. It would be a "favor to go through cartons of material. IF the Bass is a multi-thousand dollar instrument, someone MAY hunt it down for you - IF you have all you information available and a person with a desk job has the time and WANTS to put in the level of hunting necessary. Someone with an injury etc, might want to do it if it's laid out with all the info.
best of luck
Paul Tutmarc: Inventor of the Electric Bass - 1931.
1st Electric Bass "Serenader":. L.D. HEATER Co. 1948