It's a little late for this comment, but it may help somebody... When I restored my gauges, I worked the ring open as mentioned earlier. To put them together, I turned a hole in a 2x6 on the lathe (mounted to a faceplate) that the gauge fit tightly into. Then I wood clamped the assembly to the work bench. That gave me a rock solid setup, the two halves were held tightly together and the ring was fully supported. I was able to tap the ring back down with a soft punch and small hammer. If I did this more often, I would've devised a way to set the gauge in the lathe, used the tailstock to hold the halves together and rolled the ring back in place.
Hope that helps.