The headlugs are a little like those used on some Raleighs and perhaps other British brands, but it doesn't look exactly like (say) a Grand Prix or similar Carlton(and if it was a GP it might have 26tpi BB and headset threading)...could be Belgian, might even be Austrian (they also used lugs like that).
The "72" is almost certainly the angle: 72°.
That's all I got, but I'd also add I doubt it has any 531 tubing.