When I was younger I used to use one of these (Hunter brand, I think), so I know how those work:
View attachment 1716396885
One thing I've noticed in all of the newer gauges posted on this thread is that the lack the spring-loaded needle-like pin that sticks out of the mounting surface, and is used to locate the center of the axle:
View attachment 1716396886
I always thought that ensuring concentricity was important to getting accurate measurements.
Is there a different way that those other gauges do that?
Thanks,
– Eric