A similar quote was attributed to Mark Twain by Vincent Starrett in his
Foreword to THE ADVENTURES OF THOMAS JEFFERSON SNODGRASS (1928) which was a
short collection of Mark Twain letters to the KEOKUK POST newspaper.
Starrett wrote:

In some museum or other, in Havana, if memory serves, there are, according
to the early Clemens himself, two skulls of Christopher Columbus -- "one
when he was a boy and one when he was a man." (p. x).

When Caroline Harnseberger published MARK TWAIN AT YOUR FINGERTIPS (1948)
she revised the quote as:

In a museum in Havana there are two skulls of Christopher Columbus, "one
when he was a boy and one when he was a man." -- P. x - THE ADVENTURES OF
THOMAS JEFFERSON SNODGRASS (1928).

This quote falls into the "attributed" category without further
verification.

Barb