I have a toddler who won't stop interfering with me this morning so I feel like I might have missed something in this thread, but I didn't see the SWCA Database mentioned.

Personally, I find that database to be invaluable, particularly while at gunshows.

It won't give me precise information, but there are enough guns in that I can usually roughly authenticate something using the database. Typically I'm looking at slightly odd old guns, the types of guns that end up in that database. When looking at say, an old K22 with an HBH, popping into the database and finding one in a relatively close serial range with an HBH confirms that the gun may have shipped with that hammer.

Of course there are no guarantees, and just because another gun in the same general vicinity of the S/N I am looking at shipped with a certain configuration at a certain time doesn't mean the gun I'm looking at did. But the way I operate (assuming everything on a gun is incorrect and paying accordingly) the database allows me to make a slightly more educated gamble then I would otherwise be able to.

For those that need an exact date and proof there are letters and the ship date request of course, but for my gambles that database is absolutely priceless.