[SAJ]

Rotorwrench, I agree they looked to be induction hardened by the coloured rings radiating from the teeth. But the back of the ring gear was still too hard for a standard high speed drill. It was interesting to me that a cheap titanium nitride coated Chinese drill bit worked really well, and did all 4 holes with coating still visible on the drill tip when I finished. A standard countersinking bit would not cut the metal either and I had to use a tungsten carbide one.
I see in Tom Endy's post the screws look like button head -countersunk or maybe counterbored. They look neater than my ones, but I used what I had handy at the time. Mine were metric too, and I usually try not to use metric on my non-metric cars.
SAJ in NZ