one other possibility: those are some oddball tubes...still don't know what to make of it, but seems like it could be metric tubing but "ovalized', however that was generally done at the BB area and not the entire length of a tube...plus you need then an altered or custom BB shell for the non-round ends.
I'm stumped: I don't think I can solve this mystery...but I can tell you that the Reynolds decal you have for 531 3-main tubes was a style first introduced in 1981. Could be useful IF you can determine that it's a legit decal. They also did the same decal in French, but doesn't mean a French (or Belgian or Swiss) builder would have to use that decal.