Shamrock Acquisition II Inc., a subsidiary of Burbank-based Shamrock Holdings, extended its tender offer for Wherehouse Entertainment until Jan. 7 . . . Baker Hughes Inc., Houston, reported that the number of oil and gas drilling rigs working in the United States last week fell by 23 to total 1,145. A year ago, the count totaled 988 . . . The Federal Reserve said the average yield for one-year T-bills, the most popular index for making changes in adjustable rate home mortgages, fell to 7.19% last week after averaging 7.23% the week before.