As Baltimore lefthander Dave McNally was finishing his warm-ups to Catcher Andy Etchebarren one day during spring training this year, he jokingly yelled, "Short curve, Andy," and threw a ball that he had held slightly off center. He was astonished to hear Etchebarren call back, "Great slider, Dave." The Orioles' $80,000 bonus pitcher had had a slider once, but strangely he had lost it entirely six years earlier when he was playing at Elmira. Even though the abrupt disappearance of the pitch did not prevent McNally from moving up to the majors when he was just 20, it did stop him from winning more than 13 games in any of his five big-league seasons. "In all those years," said the 5'11", 195-pound Montanan, "I couldn't make the hitters swing at my curve because they knew I'd eventually pour my fastball in. Now I can keep them off balance with the slider." That is something of an understatement. Since the All-Star Game, McNally has started 15 times, has yet to lose and has set a team record of 12 straight wins, one of them a victory over Chicago last week which ran his season's mark to 20-8. If his teammates had averaged more than 1.4 runs a game in his eight losses over the first half of the season, McNally might be up with Detroit's Denny McLain challenging for 30 wins. During his streak, the Orioles have scored five runs a game and he has hardly hindered himself by hitting three home runs, one a grand slam. McNally is mystified by his batting outburst, but Manager Earl Weaver is not surprised at his pitching. " McNally," he says, "is the best lefty in the league. You look for him to win every time he goes out. He could win 20 every year."