"Antonio Vernucci" wrote in message
. ..

What amazes me is that I could not find any visible adjustment point (e.g.
a variable magnetic shunt). Evidently they were able to produce very
precise meters requiring no adjustment.


Good find. Better quality meters were rated to 1 or 2 percent at FULL SCALE.
Also, many are rated for mounting on a steel panel. Many are not so rated.
The presence of external magnetic fields WILL affect the reading.

Sometimes you just have to go with the real meter indication and re-mark the
scale to the correct numbers.
I've done that a number of times, it shows the non-linearity of lesser
quality meters.