I have measured the BB shell, it is 68 mm wide, the fork tube is 25 mm, but the inside seems to be tapered up to a point, hence accepting the 22.2 mm stem I bought it with. I partially cleaned the HS (sorry, no photos yet), it seems to be an older version of Edco headsets.
MauriceMoss proves a great point here, the clues so far point towards a Mondia frame with the Edco headset, Bocama lugs, and SN on left side of the seattube joint, one of the pictures even has the same color scheme - silver paint with blue headtube and red pinstriping (but with different lugs).
I will try removing the red and black paint and restore the original finish, maybe apply clear coat to seal the exposed steel, but it is big pain in the butt, wouldn't go for the repaint route as it would be a bit over the top.
Thank you everybody for your help, you're all great!