A little Did You Know taken from a Nikkan Sports article.
Yoshihiro Maru has the following etched into the handle of his bat: C9 B.
The C9 is for Carp and uniform number 9, or Maru. The B indicates the number of times he has made changes to his bat since he joined the team. In this case, B means he made a change to his bat once (he got a thicker handle).
Source: Nikkan Sports 5/30/2016