B is for the Base, the 2nd Letter = the month the unit was manufactured;
A=Jan,
B=Feb,
C=March,
D=April,
E=May, F=June,
G=July,
H=Aug,
I=Sept,
J=Oct,
K=Nov,
L=Dec.
The first number in the group = the year the unit was manufactured, for example BL713984 = Dec of 2007.