Thank you to all who helped me sort out how to "tie in" MT to Newton, Mass.,
where I am planning to give my "Mark Twain on Postcards" talk later this
year.  It certainly pays to ask for help.  Starting with a fast response
from Barb Schmidt (first out of the blocks as always), I got all I asked for
... and more ... and better.  Viz.:

(1) I was conflating two incidents involving two different editors of The
Atlantic;
(2) The expedition was to Concord, in a different direction from Newton
entirely; (3) And, trumping all, thanks to Vic Fischer:  MT lectured years
earlier at a church not 300 yards from the site of my planned talk.

What a group!

Henry Feldman
Newtonville, Mass.