531 Help

531 is advanced gaspipe. and was only available in small O/D's that build into a 'comfortable' frame. i.e. flexible.
it was good stuff at the time when introduced in 1935 but is essentially the same as current 'stock' 4130 steels that you would use to make a wheelbarrow handle or a piece of farm machinery out of.

4130 is chromium molybdenum = it's more like 501.
531 is manganese molybdenum.

current steels like spirit/853/631/sat-14/zero-replica/true-temper etc are superior in every way to 531 apart from not being 'awesome' in an overpriced mid range late 70's conversion frame to fit you badly from ebay kind of way.

Of course - and these tubesets are much newer, exotic and and much more expensive.