DRESBACH, Minn. — A person fishing under the Interstate 90 bridge felt a hard tug: “A super-size bass perhaps?” What he reeled in was.a pump shotgun, severely rusted and mired in mud. Sheriff’s deputies from Winon couldn;’t make out a serial number. The chamber was so rusted they couldn’t determine if it were loaded. They took the gun back to Winona in hope a that  little WD40 and elbow grease might yield a serial number to track the gun’s origins  There was no reason to suspect criminality.