So I asked a bit more about that file and even asked local knife maker (ex Yu area) for tips.
So:
1. This file is even older than I initially thought. It is from the time of former Yugoslavia, I said it's definitley over 30 years old, but it's over 40 actually.
2. Everyone says tools were of much higher quality back then.
Then I went back to the file, and this file actually still has it's bite, especially at the edges and non corroded areas (rust is only on surface).
The knife maker gave me advice to anneal it first since this steel at this hardness will be a chore to grind and if I do something wrong I could still mess up the heat treat.
I also got advice how to quench and temper, but I don't have the forge or any place to heat it to the temperature where I can go into the quench.
So I'll try to do it like the way I originally planned, if I mess it up, then I'm going to anneal it and basically do everything I want to do, and once I'm done - take it to the company that heat treats stuff for you.
It will cost me, true, but they will quench and temper it either to HRC you want or HRC they see as appropriate if you didn't state anything. One thing they will ask you is - "What steel do you have?"
-I think saying it's similar to O7 would be fine, I did more research and either OW3 or c.6441.9 would be fine, they were both called Č6441. They were used a lot, especially for tools that required high hardness and wear resistance.
This steel was even used for dies and punches.
Slovenia and Macedonia mainly produced steels like that, these steels were mass produced and their composition varied a lot. Instead of using various steels like we see today, the guys back in the day would just tweak the composition of the steels they already produce instead and/or heat treat it to another hardness.
Another role was played by politics, as communist propaganda wanted to use their own materials and were against import.
So, in a nutshell, instead of using let's say D2 for dies and A2 for punches, you had two O7 modifications heat treated to different hardness.
With that being said, all consistency goes through window and you have the elements off the charts like we can see here with numbers all over the place.
But as long as it worked for them - it's all good.
At the other hand, when it comes to this file being a file, if it was new it'd actually be a really damn good file.
All tips are welcome tho.