What an interesting measuring thingumybob
Does the anvil with the red blob on it pivot into two positions? The scaling, being red on one side implies that it refers to the measurement to the red portion of the anvil, and if the anvil doesn't pivot then the object being measured has to be an odd shape.
My first guess is that is for magnifying the measurement of something fairly thin such as sheet metal or whatever
(When I first saw it I thought that it was a jewellers gem measure, but I think the small size of the moving 'beak' precludes that)