Cracked lug-tube joint
Been looking at this frame recently and have just noticed a crack approximately 1cm long where the downtube is brazed into the lower headtube lug. It looks rusted in there, so it's probably been there for some time. There are also striations on the lug itself, where the paint seems to have cracked for some reason (maybe impact?).
Does anyone with experience of seeing this type of issue recognise the cause, severity, and possible remedy?