I've drilled it to 6.75mm (closest drill size I have) and I can see traces of thread lines. Might just get some alum and leave it to do its work. Does alum eat stainless? Ideally I'd put the caliper + alum into my ultrasonic tank. The heat and the vibes should speed it up. But not much use if alum eats stainless...
send a taper tap down it, the thread remnants should come away easily now