FOR nearly half a century since Bobby Thomson hit the home run that won the 1951 National League pennant for the New York Giants, Ralph Branca, the Brooklyn Dodgers right-hander who threw that fastball, has known that the Giants were using a high-powered telescope and a buzzer system late that season at the Polo Grounds to alert their batters to the next pitch.

''I've known it since 1954, but I never said anything,'' Branca said last night. ''I didn't want to cry over spilled milk. I became friendly with Bobby and I didn't want to demean his home run. I didn't want to cheapen a legendary moment in baseball.''

Yesterday in The Wall Street Journal, which reported the sign stealing, several '51 Giants acknowledged the presence of a telescope that was perched behind a window in Manager Leo Durocher's office in the center-field clubhouse and the buzzer system connected to the Giants' dugout and right-field bullpen.

When Thomson was asked if he knew that Branca would throw a fastball that he drilled into the lower left-field stands for a three-run homer and a 5-4 victory in the decisive third game of the N.L. playoff, he hedged, but finally said: ''My answer is no. I was always proud of that swing.''