Oddleg Calipers
From DT Online
Oddleg calipers (aka Hermaphrodite Calipers) are used mainly to scribe lines parallel to the edge of metal (or plastics) - they are used for much the same reason that a woodworker might use a Marking Gauge.
Oddlegs can also be used to find the centre of a disc by first estimating its radius then scribing a number of arcs from different points around the circumference. The various arcs will enclose a small area in which the centre must lie and so can be judged.
Note: To make scribed lines stand out more, bright steel can be coated with Marking Blue (or a felt tipped pen) whilst black steel or rusty metal can first be rubbed with chalk.