Henry, I can find nothing of this incident in Howell's  remembrances of Sam
in "My Mark Twain." There is a description of his first  meeting with Sam in
1869 in the office of James T. Fields, 124 Tremont Street,  Boston. In that
meeting Sam wore the sealskin coat made so famous. (p4)

Sam often visited Howells in his homes in Cambridge,  Boston and elsewhere,
and the favors were returned. Both men married frail  women, had daughters
die
prematurely, and shared literary interests  and achievements.

Howells does describe an incident in April, 1875 when  Sam arrived at
Howells
to travel on to Concord for the centennial celebration of  the Minute Men
battle with the British. They wanted to take the train for  Concord at the
Cambridge station but the train was packed. They snuck home and  pretended
to have
been to Concord but were found out. (pages  39-40).
Could this have been the event you recalled?

I am still in the 1860s with much of my work, but have  done some of the
Buffalo period, so have not come across much yet that might  help you. A
cursory
check of incidents involving Elinor Howells and such did not  turn up any
other
notable incidents, but if you can obtain Howell's nostalgic  work, My Mark
Twain, it may jog your memory.

David H  Fears