The Yankees have hit a goodly number of home runs this season, as you have presumably noticed. Through last night’s games the (well-nicknamed) Bombers lead the majors with 207 dingers, through game 138 of the season.  That’s a pace that would generate about 243 homers if maintained over 162 games, which is impressive but it is hardly unprecedented. A 243-homer season would tie for the 11th-most team homers in a major league season.

Where the 2012 Yankees are on record or near-record pace is in hitting home runs off of left-handed pitchers. Details after the jump.
Through last night’s game (which included three Yankee homers off of lefty starter Wei-Yin Chen), the Yanks this season have hit 72 homers off of lefties. That’s an 84.5 homer pace over a full, 162-game season. Going back to 1948, which is as far back as Baseball-Reference has records for this kind of split, here are the most team homers off southpaws that the Play Index shows for any season:

1. 1973 Braves, 85 HRs off lefties
2. 1996 Orioles, 83
T3. 2004 White Sox and 1985 Orioles, 78
T5. 2009 Yankees and 1977 Yankees, 76

Aficionados of baseball stats and/or baseball history may well recognize those 1973 Braves (managed by the then-recently-retired slugger Eddie Mathews in his only full season managing a major league team) as a familiar source of unusual home run numbers. Although that Atlanta team as a whole produced what now looks like a relatively measly 206 homers (currently tied for 99th highest team total all-time), the ’73 Braves continue to be one of only three major league teams to have three different hitters clobber 40 or more home runs in a single season. The only other teams with three 40-or-more-home-run hitters have been the Rockies of 1996 and 1997. On that Braves club, the 39-year-old Hank Aaron and the young Darrell Evans were certainly legitimate sluggers (though aided that season by a home run-favorable home park), but the biggest surprise in the 40-homer club for Atlanta that year was Davey Johnson, who never hit more than 18 home runs in any other season in his career. Davey’s become more famous as one of the great managerial turnaround-specialists in the history of the game (currently burnishing that reputation daily), but in 1973 he worked some sort of magic on himself, with Rogers Hornsby remaining to this day the only man to hit more homers in a season while in the game as a second baseman.

And along with Aaron, Evans and his other mates on the ’73 Braves, Davey contributed to the greatest team home run total against left-handed pitchers to date, or at least since 1948. Without split data before 1948, we can’t be absolutely sure the Braves hold the all-time record. On the other hand, with just one exception, the top team home run totals before 1948 tend to be lower than the top totals of more recent years, so it’s unlikely that any team before ’48 managed 85 homers off of lefties. The 1947 Giants had 221 homers, the only pre-1948 team total that would still stand out today. Retrosheet has lefty pitcher/righty pitcher split numbers for about 80% of the at-bats for that team, which show only 42 homers off of lefties for the team, so we can be pretty confident that the ’47 Giants did not reach 85 total homers off lefties. No other pre-1948 team had more than 182 total homers, so it would be very surprising if any team of that earlier era had more homers off lefties than those 1973 Braves. I’ve checked what limited splits are available for the top homer-hitting teams in the pre-1948 era (b-ref can show you how many homers a team hit in games started by a lefty), and those suggest it is extremely unlikely anyone came close to the ’73 Braves’ total for homers off left-handed pitchers.

In short, we can be pretty confident that the 1973 Braves hold the all-time team record for most homers off lefties in a single season. And that the Yankees this year are making a run at taking over that record for themselves.