[snip]
When I worked as a meter tech for EIL many years ago we calibrated iron vane meters of various types. Most of them had a moveable metal piece that could be adjusted to affect the calibration. Most of these meters were made to be opened easily for repair and adjustment and scale changing, but some (GE I think) had an aluminum band that was swaged on with a special tool to seal the meter. All iron vane meters I worked on were for AC use, but they might work on DC. I never tried. However, I think they relied on magnetic induction which is an AC phenomenon, similar to a motor.
The really cheap meters were Shurite, and they used (I think) a permanent magnet moving vane in a fixed coil. These were often DC meters used in automotive and battery charger applications. The permanent magnet might weaken with age, but I think that would make the meter read lower, rather than high. If there were no magnetism, it would read zero. All meters I worked on had springs (taut band types use the band as a spring). A weak spring could cause a high reading.
The only meters I know to be true RMS are dynamometer types, which use a moving coil as well as a fixed coil. When the coils are wired in series or parallel, they read voltage or current. When the coils are separate, the meter can be used as a wattmeter.
Of course, most such measurement is now done digitally, but it is interesting and educational to understand the older technology. Also, many of the lab grade meters were beautiful works of art, with varnished oak or bakelite cases, and carefully drawn mirror scales to reduce parallax, and to obtain accuracy to 1/4% or better.
Paul