Changing the offset and span on an analog voltmeter
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. The meter wiring based on OP's first photo and text.

How it works:


D1, D2 and D3 are Zener diodes. With the three in series as shown no current will flow through the meter coil until the voltage rises to more than 22 + 20 + 18 = 60 V.
Above this voltage the meter will swing progressively towards full scale.
From your readings it appears that the meter goes from zero to full scale with an additional 20 V.


Your modified meter specification is as follows:


Voltage span: 84 - 134 V DC.


To achieve this we need to do two things:


Add another Zener in series to raise the start voltage. You need an 80 - 60 = 24 V Zener.
Add some further series resistance.








simulate this circuit

Figure 2. Modified meter circuit.

When you provide the meter coil resistance as requested in comment #2 we can calculate that.



The coil appears to be 812 Ω. So we get full scale deflection with a total resistance of 510 + 812 = 1324 Ω at 20 V.


To change this to full scale deflection of 50 V (134 - 84) we need the total resistance to be \$ R_{TOT} = 1324 \cdot \frac {50}{20} = 3310 \; \Omega \$.
R2, therefore, should be 3310 - 1324 = 1986 Ω or 2 kΩ.