They are look the same but they are not. I tried to check from the user’s feedbacks.
These one is the cheapest and have fewer components. Maybe it’s the first generation.
Use an inductor 100µH. It has green LED power.
The left button use to turn on/off voltmeter.
The right button use to switch between input and output voltage.
There is a blue PCB version which slightly differences.
Model 2 (MH)
It shows leading zero. Use an inductor 47µH. Both input/output LEDs are red color.
Use only one button for all functions.
Short press = switch between input and output voltage.
1-2 sec = calibration
3 sec = on/off voltmeter
The PCB is bigger than another.
Use an inductor 33µH. Both input/output LEDs are red color.
Model 4 (2596-SDC)
Use an inductor 33µH. The output LED is green color.
Like the model 1, has two buttons but come with more components.
Use an inductor 33µH and input capacitor 100µF.
Both input/output LEDs are red color.