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@lae
Having looked at the photo of the frame, I'd err towards it being 531 main tubes - it has double water bottle mounts and forged dropouts, both of which indicate a reasonably good quality frame.
You could always give the main tubes a flick with a fingernail - a high ringing noise indicates thin-walled 531, whereas a dull thud indicates thick-walled high-tensile tubing. It's not exactly a reliable test, though...