Sacrifice a little bit at the end with an angle grinder to see if it's all hard or just case hardened. I'd make some angles 4 or 5 mm long to expose the core and try it with a file or hacksaw to verify whether it's hard or not. need to take the corners of the angles out too to expose the core.
From what I've seen the base material is generally pretty obdurate an any serious hardening is concentrated on the top where the holding bolt goes. Base material is still tough enough to give an end mill a hard time. I find my Elliot 10M shaper does better.
A good carbide end mill should handle it just fine. If the angled cut test mentioned above shows it is case hardened starting the cut where the softer material shows will give the cutter an easier time.
Clive