I went and looked through the L-39 manual to see if it had any information about this. It says: "Pressure correction indexes for landing at high altitude airfields, where pressure is less than 670mmHg. Indexes are moved by the knob."
The manual points to arrows 1 and 2, which I think is an error. I think arrow 2 is for alignment of the barometric pressure.
Then I noticed something about the image foxbat155 posted.
The alignment between 800m mark and 25km mark. Compare images:
Then I saw the numbers in the pressure scale. They increase going clockwise in all images except the one from our game Mig-21.
What's going on here?