"B" is also the letter prefix for parts built/created in the 50's.
C = 60's
D = 70's
etc.
It also can depend on what the complete number is,
and if it's an; engineering, casting or part number.
http://www.fordification.com/tech/partnumbers_overview.htm
https://www.google.com/search?q=decoding+ford+part+numbers&ie=utf-8&oe=utf-8
.