Listed in my 1938 Starrett catalog as a micrometer caliper gage, " this gage is specially adapted to the tire industry, in measuring tire molds and by affording greater scope than any tool of its kind made and is valuable in many other lines." The metric versions were available in 30 cm, 50 cm, 60 cm & 90 cm sizes. Ranging in prices from $16.75 - $24.00. The catalog picture shows divider type of points threaded into the tips of the jaws. Other attachments were available to assist using calipers in conjunction with the tool. # 24 A is inch series and the # 24 M is the metric version.