An example of removing a very hard screw. I dulled a couple of quality drills using a hand held drill motor on this broken off stud in a lathe bed. Only 3 studs are used and this one was so hard I thought it was a broken off form tap. The other studs were missing when I got this lathe so I did not even know what was broken off in there.
Since the first attempts failed I bolted the bed to the mill table and milled a flat area on the screw then drilled it with a carbide spade drill that I sharpened for left hand rotation. After a while the bit grabbed onto the stud and screwed it out. Carbide drills require a rigid set up or they will shatter like glass and the mill allowed a heavy accurate feed so it did not just rub on the surface and make it harder. These carbide spade drills are sold as "die drills" for tool and die makers to drill through hardened die sections.

Back to Carbon Outlaw, a close up picture of the threads would help diagnose if the screw is original. And a brush with a file will tell if it really is that hard.  Your diamond drills mention grit size so that would indicate diamond dust held on with a nickel plating like a diamond knife sharpening hone? If you drilled that with a hand drill you are a better man than me. For a original screw to be that hard would mean it was actually heat treated and missed the tempering operation. To me that would be a surprise as they ones I have seen are so soft I would not have guessed they had any treatment.

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