Mauser Caliper Graduations.

[mklotz]

OTOH, the formula for the area of a circle is pi/4 times the square of the diameter and, suspiciously, pi/4 = 0.7854

If the number on the back scale was the square of the diameter read with the caliper then multiplying the back number by 0.785 would provide the area and the area would be simply related to a standard length to provide volume. An example will help my clumsy attempt to express my thinking in prose...

Suppose the back number represents in mm² the square of the caliper measurement. Then multiplying that by 7.85 ( 0.7854 * 10 mm = 1 cm)) would give the volume per centimeter of the cylindrical wire being measured.

If one then had a table of specific weights of various metals, expressed in terms of gm/cm³, it would be easy to calculate the weight of a long length of wire.

Admittedly, just another SWAG :-)