531 Help

4130 is chromium molybdenum = it's more like 501.
531 is manganese molybdenum.
Of course - and these tubesets are much newer, exotic and and much more expensive.

The actual breakdown of 4130 alloy steel is as follows:
Carbon
0.28 - 0.33
Chromium
0.8 - 1.1
Manganese
0.7 - 0.9
Molybdenum
0.15 - 0.25
Phosphorus
0.035 max
Silicon
0.15 - 0.35
Sulphur
0.04 max

so there is molybdenum content in 4130 but as neither of us are metallurgists with access to advanced teating facilities we would not be able to tell the difference between identically dimensioned frames, one having .20% more molybdenum in it.

modern steels aren't 'exotic' they are freely available from tubing suppliers and mass manufacturers use them in frame manufacture.
there are few NOS 531 anniversary tubesets out there if you want to pay a premium for a new rare and exotic frame built with outdated materials.