@Dan is right! And here it is: two prototypes, one in chipboard and one in acrylic. These calipers were based off an image of a vintage tool I found online, and are used to automatically locate the centre between two points.
The prototype works very well for a first attempt, although as the curved arms are spread apart, their tips gradually rise and are no longer aligned with the central yellow component
I’ll be exploring a way to rectify this, possibly using a cam or pin and slot mechanism, so that as the curved arms are spread apart, the centre indicator gradually rises so ensure all three points remain in line.
If anyone has any suggestions, I’d love to hear them!