1961: 12 of 61 Maris home runs against lefty pitchers.

Roger Maris batted left handed and had trouble hitting against left handed pitchers. However in 1961 Maris did better than usual. Check his home run rate (AB/HR) against lefties. It's better than that for Mickey Mantle in his career.

Split	G	PA	AB	R	HR	AB/HR
vs RHP	135	485	413	99	49	8.43
vs LHP	83	213	177	33	12	14.75

Here are the 12 home runs Maris hit against lefties in 1961:

#car	#yr	#gm	Date	@Bat			Pitcher	Score	Inn	Out	RoB	RBI	BOP	Pos
101	4	1	1961-05-17	NYY		WSA	Pete Burnside	behind 2-8	b 8	1	1--	2	7	9
120	23	1	1961-06-17	NYY	@	DET	Don Mossi	behind 0-8	t 4	0	---	1	3	9
122	25	1	1961-06-19	NYY	@	KCA	Jim Archer	tied 2-2	t 9	0	---	1	3	9
123	26	1	1961-06-20	NYY	@	KCA	Joe Nuxhall	ahead 3-2	t 5	0	---	1	3	9
126	29	1	1961-07-02	NYY		WSA	Pete Burnside	tied 2-2	b 3	1	12-	3	3	9
134	37	1	1961-07-25 (1)	NYY		CHW	Frank Baumann	tied 0-0	b 4	1	-2-	2	3	9
139	42	1	1961-08-11	NYY	@	WSA	Pete Burnside	ahead 8-1	t 5	2	---	1	3	9
143	46	1	1961-08-15	NYY		CHW	Juan Pizarro	behind 0-2	b 4	0	---	1	3	9
144	47	1	1961-08-16	NYY		CHW	Billy Pierce	tied 0-0	b 1	1	1--	2	3	9
145	48	2	1961-08-16	NYY		CHW	Billy Pierce	ahead 2-0	b 3	2	-2-	2	3	9
150	53	2	1961-09-02	NYY		DET	Hank Aguirre	ahead 5-2	b 8	1	1--	2	3	9
152	55	1	1961-09-07	NYY		CLE	Dick Stigman	ahead 2-0	b 3	2	---	1	3	9

Maris hit three off Pete Burnside and two off White Sox ace Billy Pierce in one game, which followed one off teammate Juan Pizarro the day before.