"ABCOS15"
The letters and numbers "ABCOS15"
are the typical hand shapes formed by your non-dominant hand while signing.
Another term for non-dominant hand is your "base" hand. For two-handed
signs in which the two hands don't have the same handshape you will notice
that almost without exception the non-dominant hand is in one of these
shapes: "ABCOS15."
You see these shapes on the non-dominant signing hand when it is providing a
stationary base for your "in-motion" dominant hand.
Also, if you are right handed and are
doing a two-handed sign in which only one hand moves then the left hand will
almost always be one of those handshapes (ABCOS15) and vice versa for
left-handed people.