Backtracking somewhat -- in 1960 Milton Meltzer in MARK TWAIN HIMSELF,
p. 135 did identify the dog in the photo as "Flash" and credited the
photo to the Mark Twain Memorial in Hartford.  The edition of FAMILY
SKETCH from UCal Press (2014) edited by Benjamin Griffin (and highly
recommended) uses a photo of the family and dog on the cover and
identifies him as "Flash" on the jacket.  Also, there is a note by
Clemens (p. 96) in this latest edition in the Jumping Frog series of
works where the dog's name is transcribed as "Flash" -- Clemens's
handwriting is likely easier to read and decipher than Susy's.

Barb