It's an attachment for a dial gauge, to convert it to a spherometer. it's 30mm between the legs which you can use to calculate the radius of the sphere you're measuring. At this point its worth noting that you might have to search for the formula online.

Dial gauges seem to be a pretty uniform size, I think this one looks identical to mine, though I can't be certain. Its also in inches, so be careful with that. I've included the step file in case anything needs to be tweaked, but this model fits nicely on the spherometer's 8mm shaft.