The date thing has pretty much been beat to death here. We can only guess at it, but our guesses are much better than the published tables. If you read any of those, it'll just give you a 20 some odd year range, with your gun being at the late end of it.
I think we can narrow down the date it was in production by understanding that your gun is right at the beginning point of K22 second model production. Thats pretty well established to be 1940. Some flys in that ointment. Maybe some of them were built in 1939 and not shipped until 1940. If I were a betting man, which I'm not, I'd guess late 1939 or early 1940. To get any better date, you'll need to spend $30 with the factory for a letter.
Value is pretty regional and also depends on your defination of condition. Not long ago these guns would not sell at $250, and some were carried for a long time to realize that figure. Recently, demand has caught up with the guns out there. A good condition M&P with a 6" barrel will sell over $400. One in near mint conditon, around $700. With the box, $900 or more. That last being a collector grade gun.
I have a good friend that loves the guns. He seeks them out. He's recently found a 98% M&P target that he thinks he can steal for $800. He'll pay it if they can make the deal. That last is just for balance. The seller has been asking $1200 for a long time with no luck.
