Black & Decker Mower Insulator , Blade 241381-01 Craftsman | eBay
Blade Insulator? I could be wrong, but I looked at the motor shaft and saw the flats didn't contact the blade at all.
The 'Insulator' pressing on the blade tightly, is what keeps the blade from slipping.
If the blade hits a stump and stops, the blade should slip, (edges breaking off) while the motor keeps spinning.
The spec for the nut is 20 to 30 foot pounds.
Just a guess, but if someone torqued the nut up to 60 or 70 ft lbs, that could be problematic..
The edges breaking off, could be just a flag, to tell you the blade has slipped..
I don't know if they are even strong enough to be used as a sear-pin device.