Ye the the base part is adjustable maybe not the dovetail part it's self.
I assumed it mounted to something with a base for QCing parts or use on surface plate?
More likely to have been used to do rapid inspections of product on a moving line, either as a straight measuring device, or as a plus-minus comparator (by setting the zero point at a target dimension, and gaging the product to see if it is over or under the required thickness. The two pictures in post #2 show this.
Not so much a "Inspection" tool, as a rapid gaging device that can be used quickly and easily, esp without shutting down the production facility just to check something not very critical to see if it is wandering, or as the hairy one above says, rapid, repetitive checking of otherwise same parts.