You could actually get a decent feel with them if your part geometry was a flat plane. A lot of times it took two people to use them. One to hold the fixed jaw in place and one to do the actual measuring. They were really rigid, precise and frigging HEAVY! Tolerance would have been less than +/- .010". For above .020" tolerance we used good calibrated tape measures. Yes we really had to protect the ends on them and check them often, like every use on close work. It was an oilfield shop but we did some cool stuff that was a lot closer tolerance than you would think. We made directional drilling tools that was used primarily only on offshore platforms because it was so expensive to run in those days.
Here's an example of a smaller part. This is a transmitter housing for MWD (measurement while drilling). It also did some rudimentary formation analyzation down hole. It had gamma ray transducers, magnetic sensors, and inclinometers in it. It was made from Incoloy 901.