Quote from intel site "This letter designator identifies Pentium 4 processors that support a 533 MHz front side bus speed. For example, the Pentium 4 processor at 2.40 GHz supports a 400 MHz bus speed and the Pentium 4 processor at 2.40B GHz supports a 533 MHz bus speed." So basically the letter b means it has a faster bus speed.
I doubt that it has anything to do with bus speed. In any event, I sent Intel an e-mail asking for assistance in the matter, as the B987 doesn't even exist outside of the tech sheet for that laptop. It isn't even listed on Intel's website.