Based on the stamped drop-outs and thick rough lugs, I would say it is highly unlikely to be 531. I would question whether it is even 4130 chromoly, if it is I would guess straight gauge with hi-tensile steel stays and forks.
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1 Super Record bike, 1 Nuovo Record bike, 1 Pista, 1 Road, 1 Cyclocross/Allrounder, 1 MTB, 1 Touring, 1 Fixed gear