I made a few out of Allen wrenchs. Very tough steel - they won't break.

Stainless steel is the wrong material for these. High carbon tool steel is what you need for the strength. Allen wrench steel is perfect. Need to use a dremel tool with cutoff wheel to make the slot.

Case hardened, mild steel would not be the right approach for this tool. It needs to be high carbon tool steel and through hardened to prevent the little tabs from bending and snaping off. Case hardening is used to prevent surface wear, not to make a part stronger.

Case hardening would normally be better for a tool than untreated, but maybe not in this case. The little tangs are very thin and prone to bending. They eventually fatigue and snap off. Case hardening is going to provide very little additional core strength for these tabs. They will be hard and not wear, but they will still bend and break off. In fact, the hard outer surface will be even more prone to crack propagation thus the tool may actually end up easier to break. I suspect the ones Terry is selling now are made of a different steel than the old ones, so it won't be a direct comparison.

No I am not wrong about this one - a through hardened tool steel would be the best choice here. The ones I have built from allen wrenches would shear off a copper switch leaf before bending.