Agree with all comments, it's a weird one, and yes very lucky. They must have suffered some stress in the past of some sort. Yes, I have larger aftermarket wheels, but they are lighter and slightly more inset than standard (so I doubt much if any added strain), and yes I have hubcentric rings. All 5 nut/studs were definitely on the car when I left home ~20 minutes earlier - I'd been working on the exhaust, and getting in and out from under the car right next to that wheel, and I would have noticed. Nothing was noticeably wrong 3 weeks (and ~1500km) ago when I rotated the tyres. All 5 broke in the area where the stud is inside the disk. I guess maybe some extra stress due to overtightening sometime in the past 40 years, combined with possibly some moisture/grime and corrosion trapped in the space where stud passes through the disk have left them weakened, but to have them all go in such a short period and without some kind of feeling to warn me through the steering wheel blows my mind. I'll be replacing every stud on the car, and plan to upsize them to 1/2" while I'm at it, though I will look into the VF studs you suggest bfit.