Welcome and sorry
.
The 64-4 dates to about 1988. 30 years or more before this the serial number was stamped in several different areas on the gun but at that time it was only put on one place. Certain revolvers of this era have the serial number engraved or etched on the left frame, under the cylinder window; this would have been three letters and four digits. If there are no marks here or on the flat bottom of the grip frame, unfortunately your best recourse is to strip the parts off and turn the frame into your local law enforcement agency.