The "B" on the flat under the barrel should mean that it was originally
a blue gun. The underlying idea is that after soft fitting, the parts
were sent to finishing. The finishing was done in batches, by parts,
and not by guns. Barrels from lots of guns were finished as a batch,
the so the "B" was necessary, to make sure that a barrel was blued.

After finishing, the parts were returned to an area for hard/final
fitting. At this point, all the parts with the same serial number are
brought together, to make up a gun.

This is why a "B" under the barrel is important. The letter ought to
clarify just how the gun was originally finished.

I'd like to add a cautionary note, about the above comment. One of
the ultimate truisms about S&W is that the factory could refinish
a gun, and NO ONE could tell. This comes straight from the ultimate
authority. My word of caution is - Don't make assumptions about the
originality of the finish of a gun.

Mike Priwer