Hi Altair,
The most common spring loaded calipers are like that :


If you look carefully, the spring tends to open the caliper, and the stop is at the exterior.
Maybe I was not clear enough in my explanations, but it's what I called a stop to keep memory of an opening, and not of a closure.
I mean, once your stop is set, you can only close the caliper and it can come back to the opening when you release it, but obviously, it's not what we need for measuring a wall thickness, we need to open the caliper, before coming back to the measure !
Sorry if my wording is sometimes confusing...
Have all a nice Sunday !
Christophe