C37 is the standard label code for the post war slant grip Mk.1 box with either B4 or B21 post codes.
I have three such boxes and all show the pistol with 'Air Pistol Mark 1' on the left side of the cylinder.
It is quite common for Webley box lid illustrations not to match the box contents. The Webley Junior is a classic example as many have the extended grip model illustrated on the box, with a standard grip model inside.