These snares are reasonably common. I once found a true Anniversay snare (with the tapped Imperial lugs) covered in dust from an estate dealer and had the pleasure of restoring this drum. Your drum though with the nice Duco finish in that condition is above average. How are the metal surfaces in terms of corrosion? The nickel plating is a challenge to restore. Are you going to clean her up? I bet it will shine up pretty nicley with a gentle detailing!
I currently own a 1940 Ludwig Standard nickel over brass (see attached). The standard strainer at the time was quite an improvement over the previous incarnation. The self-aligning Imperial lugs had been around for three years in 1940 and were a huge improvement.
The difference between the 1940 and the 1938,39 models is the additional strainer "roller" portion below the Standard strainer and the snare hoop gate is extended below the level of the drum.
kellyj