The camera in question is an old Zenit 12XP. It belonged to my passed-away father-in-law.
It has shutter speed regulations: B, 30-X, 60, 125, 250, 500
"ГОСТ/ASA" (ISO): 16 * 32 * 65 * 130 * 250 * 500
And HELIOS-44M-4 2/58 lens (aperture 2-16).
He stuck a note on the back of the camera body - I guess it could be something similar to sunny 16 rule, but this camera has a built-in battery-powered light sensor. Maybe he used it when batteries were dead? Still, I don't know how he would use it, could some numbers represent hours on a sunny day?
19/65 21/90 22/130
Or maybe it was somehow used to calculate aperture in relation to distance when using flash?
The previous owner was an electrical engineer, so there might have been some in-memory calculation required in order to use it.
Does anyone have any ideas about what could it mean? I'd love to know how my father-in-law used it :)