Most Home Runs per season over fences.

See the data. (updated through 2017)

Cruise through the data yourself.  Until 1931 a ball that bounced over a fence was a home run.  I looked for both Bounced HR and IPHR  (inside the park home runs).  Some quick comments:

In 1909 Ty Cobb won the triple crown, leading AL in BA, RBI, HR.  All 9 of Cobb's HR were IPHR.  Frank Home Run Baker hit four HR, three of which went over fences to lead the AL.

In 1913 in NL Gavvy Cravath of Philadelphia beat teammate Fred Luderus 19-18.  However, Cravath hit one IPHR and FIVE Bounced HR: only 13 over the fences.  Luderus had one IPHR and two Bounced HR: 15 over the fences.

OK, you see how it works.  The most recent occurrences of HR leadership being impacted by HR not over the fence:
2013: Pedro Alvarez (Pittsburgh) one IPHR tied Paul Goldschmidt (Arizona) at 36
1984: Dale Murphy (Atlanta) one IPHR tied Mike Schmidt (Philadelphia) at 36
1982: Dave Kingman (Mets) one IPHR beat Dale Murphy (Atlanta) 37-36
1980 Ben Oglivie (Milwaukee) two IPHR tied Reggie Jackson (Yankees) at 41.

The only one between 1980 and 1952: Mickey Mantle 1958 AL.  Some digression for the Mick.

In his career Mickey Mantle hit 6 IPHR:

1953-08-07 NYY CHW Connie Johnson Yankees 6, White Sox 1
1958-05-09 NYY WSH Pedro Ramos Yankees 9, Senators 5
1958-05-20 NYY @ CHW Dick Donovan    Yankees 5, White Sox 1
1958-06-05 (1) NYY CHW Early Wynn    Yankees 12, White Sox 5
1959-05-12 NYY CLE Cal McLish    Indians 7, Yankees 6
1961-06-30 NYY WSA Dick Donovan    Yankees 5, (new) Senators 1

33 percent of Mantle's IPHR were against Dick Donovan.  All six were batting left handed.  Half in May, one third in June, early in the season.  The three in 1958 provided Mick with his margin of victory over Cleveland slugger Rocky Colavito: 42-41 in that season's HR race, a prelude to Mantle's HR battles against new Yankee teammate Roger Maris in 1960 and 1961.

Mantle's May 9 first of three 1958 IPHR in less than a month broke a 2-2 tie in the third inning at Yankee Stadium. It was off Pedro Ramos direction unkown. Albie Pearson, a natural CF, was in CF but slow Jim Lemon was in RF.

Mantle's two IPHR in 1958 against Chicago, one home and one away, only 16 days apart.  The first game was in Chicago; Jungle Jim Rivera CF, Tito Francona RF; neither known for their defense.  Rivera played less than 40% of his games in CF.  The home run was to CF in the 4th inning.  Mantle had doubled to left leading off the second.  Mantle grounded to second leading off the 6th but then Donovan intentionally walked Mantle in the 8th.

Mantle's IPHR 16 days later in Yankee Stadium had slick fielding Jim Landis in CF and Rivera in RF.  Mantle grounded out to second in the first against Wynn but lead off the third with his IPHR, again to CF.  Mantle later flied out to LF against Wynn, grounded a single to right against Fischer and was intentionally walked by Qualters.

Mick's other IPHR:
- 1953: to LEFT in third with two runners off 30 year old rookie Connie Johnson giving Yanks 3-1 lead; Jungle Jim Rivera CF; Sam Mele LF; not such good fielders; Yogi Berra then homered driving out Johnson, who had pitched in the Negro leagues 1940-1950.
- 1959: leading off third off Cal McLish "to LF (Line Drive)"; Minnie Minoso LF; Jim Piersall CF;
- 1961: off Dick Donovan (led AL in ERA, ERA+) CF; Maris preceded Mantle's IPHR in the 6th with "Ground-rule Double (Deep RF)", HR under pre-1931 rule; Willie Tasby CF, Chuck Hinton RF.

Cool stuff.