A wire blade is an old fashioned measuring instrument broadly similar to today's external gauge.
Used to determine tolerances as well as wire diameters, it features two opposing rows of jaw-shaped notches with measurement markings and tolerance limits. Like today's external gauges (shaft gauges), wire blades were used to check the geometrical tolerances of wire. The good side incorporated the acceptable maximum dimension and the scrap side the minimum dimension.
See Also
Adapted from “"We do it straight" — Wire Straightening”, p. 199 (ISBN 3-00-005897-4).