Mike,
That might explain the wedged flat latch, though I think on yours it's the one with less incline to it.
You still bought an early Airweight and it has the lock screw which is always an interesting Airweight feature, and the aluminum cylinder.
A 70s nickel Mod 40 mint condition is also a collectable gun. The reference works say nickel original run(up until the L was put at the beginning of the serial number) Centennials and Centennial Airweights are far less common than blued versions.
You're not going to shoot this gun with the alloy cylinder, I hope?
Also, don't send it to S&W for any type of repair with the alloy cylinder in it or they will return it crushed with a new steel cylinder installed(if they have any blued Mod 10s left which I doubt), on advice of their lawyers. Stick a Mod 10 cylinder on it if it ever has to go back to Springfield and leave the aluminum one at home.