I found three baseball references in my MTP Microfilm Edition Index
(work-in-progress):

                                                         Roll:Position
Burlesque of Books on Etiquette                            38:20
  29 pgs MS ORIG CU-MARK    DV 068
Advice on various subjects: At the Dog-fight (2 pgs), Wall street (1 pg),
At Poker (2 pgs), Taking a Drink (6 pgs), In the Restaurant (1 pg), At
the Baseball Game (3 pgs), At Billiards (6 pgs), At Sea (1 pg), other
fragments.  See MTB, 2:705-706.

Methuselah's Diary, second                                 39:4
  23 pgs MS ORIG CU-MARK    DV 089
Description of "...an ancient game, played with a ball..." (baseball) and
a report that Adam, the father of mankind, is coming to town.

Speech on Sandwich Islands                                 39:20
  10 pgs MS ORIG CU-MARK    DV 112
Welcome home to a baseball team returning from a world tour by way of the
Sandwich Islands. 1889.  See Mark Twain Speaking (Fatout), 244-247.

Paul Berkowitz