Could "590" be the frame size i.e.: 59cm (did you measure the seat tube)?
Was the BB threading ever determined? if you got the NDS cup out yourself did it unscrew counter clockwise (in the usual fashion of 99% of all screws), and since you are discovering faint markings is there ANYthing on either of the BB cups? I
If I was betting (not) at this juncture I would bet on "French", but Swiss is still possible and to lesser degrees Austrian or even Belgian.
One more (set) of clues, since you have that digital caliper measure the ODs of all 3 main tubes: Seat, Down and Top which will at least confirm that this is metric gage tubing