Alan,

Yes,  the family really wrote it both ways -- Susy/Susie. " Record of
the Small Foolishnesses" spells it Susie.  At some point in time
Olivia Susan Clemens (referred to by the family in early letters,
notes, etc. as Susie) wanted to change the spelling to Susy.  That is
how her manuscript of the biography of her father "Mark Twain" is
presented. Throughout "Small Foolishnesses," it is rendered as Susie.

Barb