She asked if it is flat or curved?
It is completely flat and approximately 21cm (8 and 5/16 inches) long.
Ive found examples online of pointed busks, rounded busks, flat-edged busks, and pieces that combine these shapes.
Btw... what an amazing comb and photo!! So wonderful to have such an heirloom! And the resemblance to the one your great-great grandmother wore is uncanny!