SWINGING A 75-YEAR-OLD mallet from the Falstaff Museum of Brewing, President Joseph Griesedieck seals the 7,000,000th barrel of beer porduced by the company in 1966. (Since the standard of measure in the brewing industry is a 31-gallon barrell, it takes tow like the half-barrel pictured to equal a barrel.) Observing the event is Charles Mitchell of the brewing department at Plant Ten, one of Falstaff's two breweries in St. Louis. The shipment of the 7,000,000th barrel on Dec. 29 (which includes sales of the firm's Narrangansett subsidiary) marked a new high in annual barrelage sales for Falstaff, which operates nine breweries and a malting plant, and sells its products in 40 states and numerous overseas markets.