Gauge Clean-up (Ferranti) and testing

I was just going to show the clean-up of one gauge but I've added this one as well because it seems interesting. 

On initial inspection this appeared to be a Ferranti Voltmeter reading 0-500 volts. But during dismantling, cleaning and testing it was found to be something quite different. Firstly opening this meter was different, two screws allowed the face plate and glass to be removed but even after this it proved to be impossible to remove the electrical component. It was also apparent that the dial had been sealed with rubber to prevent water or dust ingress suggesting this gauge had some industrial or outdoor use. With the dirty glass removed it was apparent that everything was not what it seemed. Beneath the handwritten word "VOLTS" it read Milliamperes although this had clearly been scrubbed off, it was also obvious that all the numbers around the dial had been similarly altered. 

Therefore I decided to remove the dial to find out more. The information on the back of the dial showed that it was indeed built as an ampmeter not a voltmeter and was manufactured in 1956.  I progressed cleaning and re-assembling the gauge as before. 

I then decided to test both gauges to see if they worked. This was done using a bench power supply but could be done using batteries. Using a power supply you should insure it has current control as well as voltage as other wise you may end up frying your gauges. I picked up this power supply off Sheffield market about two years ago for three pounds, the man selling it had no clue what it was or what it was worth. It was connecting the Ferranti gauge up to this power supply that gave me my second surprise,  I set the power supply to 4 volts and expected the needle to hardly move (the gauge claiming to measure up to 500 volts) the gauge immediately maxed out. I checked that it was all set up right and tried again, same result. It is clear that for whatever reason someone decided to re-mark this gauge to give a completely false reading. 

Anyone know why?

Happily both gauges move when hooked up to a current meaning it may be possible to have them "working" (needles sweeping) on the front of the PC.