Well that's the second 64-5 prefix I've seen on the forum tonight that is not listed in SCSW. And both began with B.

Until someone more knowledgeable chimes in (are you there, Kernel?), here's a few things to look at to narrow the range;

Forged trigger (not MIM), pre-1997
Uncle mikes grips, 1994 or later
Round-butt, 1995 or later
Serrations on grip frame front & backstrap, probably 1995 or 1996.