Wah. That story was a load of garbage. Thomson was the hottest hitter in the league and owned Branca all years.
Home road splits might suggest otherwise... the system was supposedly in use much of the year... But the Dodgers didn't want Branca to face Thompson. Erskine couldn't get loose in the bullpen and Branca was the only pitcher available, as Newcombe was toast by then. Might have been the time to issue an intentional/unintentional wallk tho'...
Thomson hit better away from home. The 1951 Giants hit for more power at home (their BA/OBP numbers were the same each way) but they hit much better in the 1st half than the 2nd, which is when people usually ascribe the whole sign stealing system to have been used. Their three best hitting months were May, June and July, not August and September/October, which is when they went 37-7. The reason they won the pennant was that in August and September/October their pitching got into shape and Maglie, Jansen and Koslo were red-hot. Their staff ERA in September was 2.58, which is awesome. Thomson had a 1.350 OPS in September, hitting .440 with 9 home runs. He was having one of the greatest hot streaks in baseball history, had owned Branca (7/18 in 50/51 with 3 home runs) and Branca was a gopher ball artist. He'd given up 24 home runs in 142 innings the year before and 19 innings in 204 innings in 1951, which is better but still not great. So yeah, while there's no doubt that the sign-stealing happened, there's very little evidence of it having a positive effect on the Giants (more evidence of a negative effect), virtually no evidence of it having a positive effect on Thomson, and plenty of evidence that a gopher ball artist facing the hottest hitter in baseball didn't need sign stealing to give up a home run in this situation. It's a fiction invented by Prager to sell books. It's telling that a contemperary, namely Branch Rickey, called it the worst managerial decision in the history of baseball.
I meant Dodger pitchers against Giant hitters overall home/vs road-- there might be enough sample there to suggest something either way. Should have italicized "might"-- sorry. Not sure what Rickey expected as an alternative... leave Newk in? He wasn't a fan of the intentional walk IIRC.